Enable coverage for cython code.

[rhpvorderman]

• codecov.yml with min 90% coverage and min 90% patch coverage.
• Make sure coverage plugin works correctly with cython code. See also Fix coverage in src layout with test. cython/cython#3831.
• Make sure igzip main code is properly tested.

[rhpvorderman]

Changed issue name because now only cython code is not covered.

[rhpvorderman]

I think this can be done by extending the cython plugin and using that as a plugin.

[rhpvorderman]

No longer relevant. Since this is a C extension now.