Investigate "Check if generated files are up to date" failures

[ericsnowcurrently]

https://bugs.python.org/issue47146

[gvanrossum]

For example, https://github.com/python/cpython/runs/5719012664?check_suite_focus=true

I believe @markshannon and @brandtbucher were hypothesizing that make -j8 somehow triggers this type of failure, suggesting a missing dependency in the Makefile.

[ericsnowcurrently]

python/cpython#32162

[ericsnowcurrently]

Per python/cpython#32134, looks like that fixed it.

[arhadthedev]

@ericsnowcurrently python/cpython#32162?

[ericsnowcurrently]

correct