Pin to Coverage 4 for Python 3.6-3.7 on Windows

[hugovk]

For some reason, the new Coverage.py version 5 is failing on Windows for Python 3.6 and 3.7.

Run coverage run tests/test_black.py
Traceback (most recent call last):
  File "tests/test_black.py", line 1788, in <module>
    unittest.main(module="test_black")
  File "c:\hostedtoolcache\windows\python\3.6.8\x64\lib\unittest\main.py", line 64, in __init__
    self.module = __import__(module)
ModuleNotFoundError: No module named 'test_black'
##[error]Process completed with exit code 1.

• https://github.com/psf/black/runs/371602079

It's passing on Windows for 3.8, and all Python versions on Ubuntu and macOS.

Here's an older build that passed for everything, before Coverage.py 5 came out.

• https://github.com/psf/black/runs/343607045

This is a workaround to get the CI back to green.

[hugovk]

Good news, this has been fixed in Coverage.py 5.0.3:

nedbat/coveragepy#862 (comment)

Restarting builds should fix the Windows issue.