Automatically add no-cover to TYPE_CHECKING blocks

[dmontagu]

Change Summary

Adds if TYPE_CHECKING: to the exclude_lines for coverage so that the # pragma: no cover can be removed everywhere it occurs next to an if TYPE_CHECKING: block.

Note: this is safe since code inside such blocks should never be executed except during static analysis.

Checklist

• Unit tests for the changes exist
• Tests pass on CI and coverage remains at 100%
• Documentation reflects the changes where applicable
• changes/<pull request or issue id>-<github username>.rst file added describing change
  (see changes/README.md for details)

[dmontagu]

I noticed this while working on #873, but assumed it would be better to put it in a separate PR.

[codecov]

Codecov Report

Merging #874 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #874   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          16     16           
  Lines        2716   2716           
  Branches      518    518           
=====================================
  Hits         2716   2716

Impacted Files	Coverage Δ
pydantic/color.py	100% <ø> (ø)	⬆️
pydantic/types.py	100% <ø> (ø)	⬆️
pydantic/dataclasses.py	100% <ø> (ø)	⬆️
pydantic/fields.py	100% <ø> (ø)	⬆️
pydantic/schema.py	100% <ø> (ø)	⬆️
pydantic/class_validators.py	100% <ø> (ø)	⬆️
pydantic/typing.py	100% <ø> (ø)	⬆️
pydantic/utils.py	100% <ø> (ø)	⬆️
pydantic/networks.py	100% <ø> (ø)	⬆️
pydantic/validators.py	100% <ø> (ø)	⬆️
... and 1 more

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[samuelcolvin]

👍 very clever/how was I too stupid to think of this before?