sagemathgh-36032: Fix pycodestyle warning E721
    
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Fix the warnings coming from pycodestyle E721, "E721 do not compare
types, for exact checks use `is` / `is not`, for instance checks use
`isinstance()`".

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Mostly I've made changes like `type(x) == type(y)` to `type(x) is
type(y)`, also adding some uses of `isinstance`. When I run `pycodestyle
--select=E721 --statistics src/sage/` I now get no hits.

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URL: sagemath#36032
Reported by: John H. Palmieri
Reviewer(s): Matthias Köppe

sagemathgh-36036: Fix E721 warnings for .pyx files
    
Fix E721 warnings for .pyx files

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pycodestyle reports: "E721 do not compare types, for exact checks use
`is` / `is not`, for instance checks use `isinstance()`." So replace
`type(x) == str` with `isinstance(x, str)`, replace `type(x) != type(y)`
with `type(x) is not type(y)`, etc. This is a followup to sagemath#36032 which
dealt with .py files.

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- [ ] I have updated the documentation accordingly.

### ⌛ Dependencies

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URL: sagemath#36036
Reported by: John H. Palmieri
Reviewer(s): John H. Palmieri, Matthias Köppe