[4.6] fixes for python4

[asottile]

Detected using flake8-2020

[nicoddemus]

I doubt we will be supporting pytest 4.6 by the time python 4.0 comes around, but of course it doesn't hurt and I guess you are also using this to test flake8-2020. 😁

Thanks!

[codecov]

Codecov Report

Merging #5801 into 4.6-maintenance will increase coverage by 0.27%.
The diff coverage is 100%.

@@ Coverage Diff @@ ## 4.6-maintenance #5801 +/- ## ================================================== + Coverage 95.92% 96.2% +0.27%  ================================================== Files 115 115 Lines 26553 26553 Branches 2615 2615 ================================================== + Hits 25472 25544 +72  + Misses 755 703 -52  + Partials 326 306 -20

Impacted Files	Coverage Δ
testing/acceptance_test.py	98.02% <ø> (+0.98%)	⬆️
src/_pytest/pastebin.py	90.76% <ø> (ø)	⬆️
testing/python/raises.py	94.87% <ø> (ø)	⬆️
testing/test_warnings.py	98.9% <100%> (ø)	⬆️
testing/test_pastebin.py	100% <100%> (ø)	⬆️
src/_pytest/pytester.py	91.66% <100%> (+0.42%)	⬆️
testing/test_monkeypatch.py	98.98% <100%> (ø)	⬆️
src/_pytest/fixtures.py	97.55% <0%> (+0.27%)	⬆️
testing/test_capture.py	99.25% <0%> (+0.29%)	⬆️
testing/test_terminal.py	99.84% <0%> (+0.31%)	⬆️
... and 17 more

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