Upgrade to latest version of coverage (#262)

The issue that was introduced in coverage 6.3 has been fixed:
nedbat/coveragepy#1310 (comment).
We can't just upgrade to `coverage` or `coverage>=6.4` because the
former could install bad versions of coverage (e.g. 6.3), and the latter
is unsupported on Python 3.6, which we want to continue supporting.

This patch just bans coverage 6.3 / 6.3.x, so with Python 3.7+, we'll
use the latest version of coverage, and with Python 3.6, we'll use the
latest supported version of coverage that's not a 6.3 release.

requirements-dev.txt

@@ -1,5 +1,5 @@
 # Python dependencies for development
-coverage<6.3
+coverage != 6.3, != 6.3.*
 pre-commit
 pytest
 pytest-cov