chore: cleanup and harden pytest config

[henryiii]

Minor cleanup, and make sure pytest is strict.

[mayeut]

It seems the warnings (turned in error) comes from

cibuildwheel/cibuildwheel/docker_container.py

Lines 96 to 107 in b52698d

	def __exit__(
	self,
	exc_type: Optional[Type[BaseException]],
	exc_val: Optional[BaseException],
	exc_tb: Optional[TracebackType],
	) -> None:
	self.bash_stdin.close()
	self.process.terminate()
	self.process.wait()
	assert isinstance(self.name, str)

adding the following gets rid of the error.

self.bash_stdout.close()

I wonder if terminating differently might help with #966 race conditions. Something like:

self.bash_stdin.write(b"exit 0\n")
self.bash_stdin.flush()
self.process.wait()
self.process.stdin.close()
self.process.stdout.close()

[henryiii]

I've implemented something close to that, thoughts?