Remove codecov from requirements.txt (#31)

https://pypi.org/codecov no longer exists because it was deleted from PyPi. https://github.com/codecov/codecov-python indicates the project is deprecated.

Author: wbarnha

requirements-test.txt

@@ -5,7 +5,6 @@ flake8
 black
 isort
 pytest-cov
-codecov
 mypy
 gitchangelog
 mkdocs