Upgrade to pex 1.4.8 and eliminate workarounds.

[jsirois]

Pex now allows us to float setuptools high and run on python 3.7 and
it supports sane platform expansion as well as graceful handling of
non-standard setuptools platform reporting for Apple system
interpreters allowing us to eliminate several workarounds.

We also pickup a fix for concurrent pex extraction which is particularly
useful when running pants from a pex using PEX_FORCE_LOCAL.

Fixes #5922
Fixes #6363
Fixes #6397

[jsirois]

Eeenteresting!: nedbat/coveragepy#715

I can hack around this. Fix forthcoming.

[Eric-Arellano]

Great that this closes a couple issues and kills off so much code! Thank you John.

I see you are working on fixing coverage library itself. If you want to merge this before that goes through, no worries on waiting for upstream. Seems like that could be a followup PR.

[jsirois]

I see you are working on fixing coverage library itself. If you want to merge this before that goes through, no worries on waiting for upstream. Seems like that could be a followup PR.

Yeah - I will not block. I think Ned takes a pretty conservative approach, especially when there is a workaround. For example:

pants/src/python/pants/backend/python/tasks/coverage/plugin.py

Lines 34 to 47 in 0390517

	# TODO(John Sirois): Kill the workaround overrides below if there is a useable upstream
	# resolution to:
	# https://bitbucket.org/ned/coveragepy/issues/646/modifying-coverage-reporting-for-python
	@property
	def parser(self):
	if self._parser is None:
	self._parser = PythonParser(filename=self.filename)
	self._parser.parse_source()
	return self._parser
	def no_branch_lines(self):
	return self.parser.lines_matching(join_regex(DEFAULT_PARTIAL[:]),
	join_regex(DEFAULT_PARTIAL_ALWAYS[:]))

[stuhood]

Awesome!

[jsirois]

... and bit by an unrelated jupyter failure which I was working on independently anyhow:

                     E   	Exception message: Could not satisfy all requirements for prompt-toolkit<2.0.0,>=1.0.4:
                     E   	    prompt-toolkit<2.0.0,>=1.0.4(from: ipython<6), prompt-toolkit<2.1.0,>=2.0.0(from: jupyter==1.0.0->jupyter-console)

Fix forthcoming for that in #6600; then I'll rebase here to pick up green CI.