tests: remove deprecated test-check-py3-compat.t

If our current source files were not compatible with Python 3, we would know by
now.

This check has not been relevant for a couple of years now and we can safely
remove it.

contrib/python3-whitelist

@@ -74,7 +74,6 b' test-check-execute.t'
 test-check-help.t
 test-check-interfaces.py
 test-check-module-imports.t
-test-check-py3-compat.t
 test-check-pyflakes.t
 test-check-pylint.t
 test-check-shbang.t