devel/py-codecov: Add test support

• Switch to USE_GITHUB
• Add TEST_DEPENDS
• Allow concurrent installation (USE_PYTHON=concurrent)
• Add do-test target
• Add a comment to explain why some test cases has been removed in patch-tests_test.py
• Bump PORTREVISION

Reviewed by: koobs
Differential Revision: https://reviews.freebsd.org/D10873