To make sage a namespace package, we need to get rid of src/sage/__init__.py.

It currently contains:

• definition of __all__ (no longer needed; has no effect on completion, neither in plain python3 nor IPython)
• definition of __version__ (unused)
• preloading of zlib (introduced in Wrong zlib library might be loaded #23122)
• Move load_ipython_extension from sage.__init__ to sage.repl.__init__ #32489: load_ipython_extension (introduced in Do not import IPython at startup #18726): This function is defined in module sage so that %load_ext sage works (see http://www.slabbe.org/blogue/categorie/ipython/); we forgot to define the corresponding function that would make %unload_ext sage work
• Remove monkey patching of inspect.isfunction in sage.__init__ #32479: monkey patching of inspect.isfunction to support Cython functions (introduced in Add IPython support for Cython functions #25373)
• monkey patching of importlib.machinery.ExtensionFileLoader (introduced in Fix Cython tracebacks on Python 3 #24681); the quest to get an upstream fix merged was apparently abandoned - bpo-32797: linecache should search for sources if loader doesn't find them python/cpython#6653
• monkey patching of sqlite on Cygwin (introduced in Cygwin: system sqlite3 modifies DLL search order #30157)

In this ticket we remove the ones that are not needed any more, and some others to another module init. #32479 and #32489 will take care of the leftovers.

CC: @kiwifb @jhpalmieri @embray

Component: refactoring

Author: Matthias Koeppe

Branch/Commit: af6642f

Reviewer: François Bissey

Issue created by migration from https://trac.sagemath.org/ticket/31420