1/X Fix check_untyped_defs = True mypy errors

[bluetech]

check_untyped_defs = True instructs mypy to also check bodies of functions which are not annotated (doc).

I would like to enable this option because it provides some internal dogfooding of the type annotations as they are added. For example, we added type annotations for pytest.raises, but since all of the tests are untyped, the tests weren't checked against it. With check_untyped_defs = True, there is at least some basic coverage (number of parameters, parameter names, etc.).

There are a few hundred such errors, mostly mypy complaining that it can't infer the type of empty containers, but some other minor stuff as well. This PR fixes the error for a first batch of modules. Each commit handles a separate module.

[bluetech]

Updated to fix a couple CI failures.

[blueyed]

@bluetech
Is that ready from your side?
Needs to be rebased.
(I would like to have it squashed also, but can see that it might be useful to have it per file/module)

[blueyed]

Oh, it does not enabled check_untyped_defs = True yet..

[bluetech]

Is that ready from your side?
Needs to be rebased.
(I would like to have it squashed also, but can see that it might be useful to have it per file/module)

I rebased, it's ready from my side. I don't mind if it's squashed - I mostly split it for reviewing. Let me know if you want me to do it.

Oh, it does not enabled check_untyped_defs = True yet..

Yes, I didn't want to send too much in one PR. I will follow up with more.

[blueyed]

@bluetech
Ok, please squash and force-push then.

[codecov]

Codecov Report

Merging #5673 into features will decrease coverage by 0.04%.
The diff coverage is 99.32%.

@@             Coverage Diff              @@
##           features    #5673      +/-   ##
============================================
- Coverage     96.25%   96.21%   -0.05%     
============================================
  Files           117      117              
  Lines         25948    25993      +45     
  Branches       2497     2497              
============================================
+ Hits          24977    25008      +31     
- Misses          667      677      +10     
- Partials        304      308       +4

Impacted Files	Coverage Δ
src/_pytest/assertion/rewrite.py	96.35% <100%> (-0.51%)	⬇️
src/_pytest/config/__init__.py	94.5% <100%> (+0.06%)	⬆️
src/_pytest/reports.py	95.07% <100%> (ø)	⬆️
src/_pytest/runner.py	98.06% <100%> (+0.06%)	⬆️
src/_pytest/mark/structures.py	92.44% <100%> (ø)	⬆️
src/_pytest/config/argparsing.py	88.46% <100%> (+0.25%)	⬆️
src/_pytest/nodes.py	95.73% <100%> (+0.19%)	⬆️
src/_pytest/_code/source.py	91.42% <100%> (ø)	⬆️
src/_pytest/mark/evaluate.py	96.7% <100%> (+0.03%)	⬆️
src/_pytest/_code/code.py	94.61% <100%> (+0.04%)	⬆️
... and 8 more

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[bluetech]

@blueyed I squashed all of the commits except for the first one.

[blueyed]

I've quickly compared the old and new number of issues with --check-untyped-defs: 219 before, 134 after.

[bluetech]

I merged this, thanks everyone. Next parts will come soon.