assertion.rewrite: use ast.NameConstant(None) directly

[blueyed]

ast.parse("None") was added/used in 3e6f1fa for differences between
Python 2/3, but we do not support py2 anymore.

[bluetech]

I also made this change in #5673, so LGTM. I think using constants for this is a bit much, but OK either way.

[codecov]

Codecov Report

Merging #5732 into features will increase coverage by 0.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##           features    #5732      +/-   ##
============================================
+ Coverage     96.25%   96.27%   +0.01%     
============================================
  Files           117      117              
  Lines         25895    25896       +1     
  Branches       2494     2494              
============================================
+ Hits          24926    24931       +5     
+ Misses          666      663       -3     
+ Partials        303      302       -1

Impacted Files	Coverage Δ
src/_pytest/assertion/rewrite.py	96.86% <100%> (ø)	⬆️
testing/test_assertrewrite.py	85.38% <0%> (+0.13%)	⬆️
testing/test_pytester.py	89.53% <0%> (+0.29%)	⬆️
src/_pytest/pastebin.py	91.22% <0%> (+3.5%)	⬆️

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[blueyed]

Thanks for the review!

I think using constants for this is a bit much, but OK either way.

Not sure either, but thought that this gets called/used a lot after all.

[bluetech]

This seems trivial enough for me to merge, so I did. Also to avoid conflict when rebasing the above mentioned PR.