Rework _WarningStream to behave closer to io.StringIO()

[miketheman]

As discussed in pypa/readme_renderer#231

Here's the custom class:

twine/twine/commands/check.py

Lines 51 to 71 in a6dd69c

	class _WarningStream:
	def __init__(self) -> None:
	self.output = io.StringIO()
	def write(self, text: str) -> None:
	matched = _REPORT_RE.search(text)
	if not matched:
	self.output.write(text)
	return
	self.output.write(
	"line {line}: {level_text}: {message}\n".format(
	level_text=matched.group("level").capitalize(),
	line=matched.group("line"),
	message=matched.group("message").rstrip("\r\n"),
	)
	)
	def __str__(self) -> str:
	return self.output.getvalue().strip()

@bhrutledge has some ideas on how to replace the usage to better reflect the future interaction of twine check and readme_renderer when docutils doesn't pass a warning, but readme_renderer does.