bd7e530 This PR adds initial support for type hints checking in python scripts. (Kiminuo)

Pull request description:

  This PR adds initial support for type hints checking in python scripts.

  Support for type hints was introduced in Python 3.5. Type hints make it easier to read and review code in my opinion. Also an IDE may discover a potential bug sooner. Yet, as PEP 484 says: "It should also be emphasized that Python will remain a dynamically typed language, and the authors have no desire to ever make type hints mandatory, even by convention."

  [Mypy](https://mypy.readthedocs.io/en/latest/index.html) is used in `lint-python.sh` to do the type checking. The package is standard so there is little chance that it will be abandoned. Mypy checks that type hints in source code are correct when they are not, it fails with an error.

  **Notes:**

  * [--ignore-missing-imports](https://mypy.readthedocs.io/en/latest/command_line.html#cmdoption-mypy-ignore-missing-imports) switch is passed on to `mypy` checker for now. The effect of this is that one does not need `# type: ignore` for `import zmq`. More information about import processing can be found [here](https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports). This can be changed in a follow-up PR, if it is deemed useful.
  * We are stuck with Python 3.5 until 04/2021 (see https://packages.ubuntu.com/xenial/python3). When Python version is bumped to 3.6+, one can change:
      ```python
      _opcode_instances = []  # type: List[CScriptOp]
      ```
      to
      ```python
      _opcode_instances:List[CScriptOp] = []
      ```
      for type hints that are **not** function parameters and function return types.

  **Useful resources:**

  * https://docs.python.org/3.5/library/typing.html
  * https://www.python.org/dev/peps/pep-0484/

ACKs for top commit:
  fanquake:
    ACK bd7e530 - the type checking is not the most robust (there are things it fails to detect), but I think this is worth adopting (in a limited capacity while we maintain 3.5 compat).
  MarcoFalke:
    ACK bd7e530 fine with me

Tree-SHA512: 21ef213915fb1dec6012f59ef17484e6c9e0abf542a316b63d5f21a7778ad5ebabf8961ef5fc8e5414726c2ee9c6ae07c7353fb4dd337f8fcef5791199c8987a

5d77549 doc: Add mypy to test dependencies (Hennadii Stepanov)
7dda912 test: Do not swallow flake8 exit code (Hennadii Stepanov)

Pull request description:

  After #18210 the `flake8` exit code in `test/lint/lint-python.sh` just not used that makes the linter broken.

  This PR:
  - combines exit codes of `flake8` and `mypy` into the  `test/lint/lint-python.sh` exit code
  - documents `mypy` as the test dependency

ACKs for top commit:
  MarcoFalke:
    Approach ACK 5d77549, fine with me
  practicalswift:
    ACK 5d77549

Tree-SHA512: e948ba04dc4d73393967ebf3c6a26c40d428d33766382a0310fc64746cb7972e027bd62e7ea76898b742a656cf7d0fcda2fdd61560a21bfd7be249cea27f3d41

Summary:
These tests no longer work, and they test trivial stuff that is already
tested in every single existing functional test (starting and stopping
nodes...)

I noticed them when testing `mypy`, a lint tool that will be introduced when backporting [[bitcoin/bitcoin#18210 | core#18210]]

See D237

Test Plan: `ninja check-functional`

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9271

Summary:
> Support for type hints was introduced in Python 3.5. Type hints make it easier to read and review code in my opinion. Also an IDE may discover a potential bug sooner. Yet, as PEP 484 says: "It should also be emphasized that Python will remain a dynamically typed language, and the authors have no desire to ever make type hints mandatory, even by convention."
>
> Mypy is used  to do the type checking. The package is standard so there is little chance that it will be abandoned. Mypy checks that type hints in source code are correct when they are not, it fails with an error.
>
> Useful resources:
>
> * https://docs.python.org/3.5/library/typing.html
> * https://www.python.org/dev/peps/pep-0484/

Note: Core still depended on python 3.5 when this was merged, so they had to use old style "type comments". With Python 3.6, it becomes possible to use the typehint syntax for variables and attributes. Unfortunately Postponed Evaluation of Annotations is only available starting from python 3.7, so for now  e have to use quotes when annotating types (classes) that are only defined later in a module.

This is a backport of [[bitcoin/bitcoin#18210 | core#18210]]

Depends on D9270

Test Plan:
`ninja check-functional`

```
sudo arc liberate
arc lint --trace
```

Add an intentional tyephint mistake:
```
diff --git a/test/functional/test_framework/script.py b/test/functional/test_framework/script.py
index c3d39d8..76179e742 100644
--- a/test/functional/test_framework/script.py
+++ b/test/functional/test_framework/script.py
@@ -23,7 +23,7 @@ from .messages import (
 )

-MAX_SCRIPT_ELEMENT_SIZE = 520
+MAX_SCRIPT_ELEMENT_SIZE: bool = 520
 OPCODE_NAMES: Dict["CScriptOp", str] = {}

```

Run `arc lint` and check that it catches the problem:
```
>>> Lint for test/functional/test_framework/script.py:

   Error  () mypy found an issue:
    Incompatible types in assignment (expression has type "int", variable has
    type "bool")

              23 )
              24
              25
    >>>       26 MAX_SCRIPT_ELEMENT_SIZE: bool = 520
                ^
              27 OPCODE_NAMES: Dict["CScriptOp", str] = {}
              28
              29
```

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9273