Fix E721 warnings for .pyx files

[jhpalmieri]

Fix E721 warnings for .pyx files

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 #36032 which dealt with .py files.

📝 Checklist

• The title is concise, informative, and self-explanatory.
• The description explains in detail what this PR is about.
• I have linked a relevant issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation accordingly.

⌛ Dependencies

[jhpalmieri]

Thank you for the suggestions; I've implemented them.

[jhpalmieri]

I'm not sure how much of an effect "Resolve conversation" has, but it's fun to click buttons...

[mkoeppe]

I'm not sure how much of an effect "Resolve conversation" has

It just collapses the comments for all readers. Good idea to do when a comment has been taken care of and you're not adding a response.

[jhpalmieri]

Thanks! I will rebase on the next beta when it's released.

[jhpalmieri]

This merges cleanly with 10.1.beta9 as is.

[vbraun]

Doesn't build, but somehow has positive review

[mkoeppe]

Indeed

[sagelib-10.1.beta8]     [7/9] Cythonizing sage/rings/integer.pyx
[sagelib-10.1.beta8]     [9/9] Cythonizing sage/structure/category_object.pyx
[sagelib-10.1.beta8] 
[sagelib-10.1.beta8]     Error compiling Cython file:
[sagelib-10.1.beta8]     ------------------------------------------------------------
[sagelib-10.1.beta8]     ...
[sagelib-10.1.beta8]             if isinstance(m, Integer):
[sagelib-10.1.beta8]                 elog = self.exact_log(m)
[sagelib-10.1.beta8]                 if elog == -sage.rings.infinity.infinity or m**elog == self:
[sagelib-10.1.beta8]                     return elog
[sagelib-10.1.beta8] 
[sagelib-10.1.beta8]             if (isinstance(m, Rational):
[sagelib-10.1.beta8]                                       ^
[sagelib-10.1.beta8]     ------------------------------------------------------------
[sagelib-10.1.beta8] 
[sagelib-10.1.beta8]     sage/rings/integer.pyx:2860:35: Expected ')', found ':'
[sagelib-10.1.beta8]     Traceback (most recent call last):

[github-actions]

Documentation preview for this PR (built with commit d57321a; changes) is ready! 🎉