Why is pip installing "Twisted[tls]>16.4" when Tahoe-LAFS asks for
"Twisted[tls,conch] >= 18.4.0"?

We're abusing combine to rewrite paths in the coverage data.  combine isn't
intended to be used with only one coverage data file named ".coverage".  So
enable parallel mode so we have only one coverage data file with the naming
scheme "combine" expects.

Maybe this gets us working path rewriting on Windows,

Something fails with "No usable data files" on CI, not sure what

We should get it from setup.cfg dependencies later

Get rid of the -vvv and give the steps better names

They have no metadata.  They lose information about extras.  They break installation.

So it doesn't squash a failure result from the test suite