Deprecate scrapy.utils.py36 module

[elacuesta]

Now that the minimum Python version is 3.6, there is no need to keep this helper in a different file and catch the SyntaxError when importing.

[elacuesta]

inspect.isasyncgen was added in Python 3.6

[wRAR]

This is actually done in #4467 (not sure why I didn't think of extracting this change), though the function is moved to scrapy.utils.asyncgen. That module will get one more function, as_async_generator, but I guess it's fine to put both into scrapy.utils.python now that we don't need this separation because of 3.5.

[elacuesta]

Ohh 🤦
Well, I guess we can still work with this PR, since it's smaller and easier to merge than #4467.
Actually, I was hesitant about putting it under utils.python, if there are going to be more functions I think I like a new submodule instead.

[wRAR]

Well, I think it's good that this will be merged separately as it reduces the diff and works fine as a separate change.

[codecov]

Codecov Report

Merging #4900 (fe8bb6b) into master (e17c890) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #4900      +/-   ##
==========================================
+ Coverage   87.87%   87.89%   +0.02%     
==========================================
  Files         160      161       +1     
  Lines        9756     9757       +1     
  Branches     1437     1437              
==========================================
+ Hits         8573     8576       +3     
+ Misses        926      924       -2     
  Partials      257      257              

Impacted Files	Coverage Δ
scrapy/utils/asyncgen.py	100.00% <100.00%> (ø)
scrapy/utils/py36.py	100.00% <100.00%> (ø)
scrapy/utils/spider.py	81.81% <100.00%> (+4.04%)	⬆️