Run unit tests with expected 98% coverage

The Pyhton3 unit tests can't run the code in `__init__.py` which
reports to Pyhton 2 that this codebase is now Python 3 only.
Hence the level for complete test coverage has been reduced form
100% to 98%.

    Signed-Off-By: Aidan Delaney <adelaney21@bloomberg.net>

Author: AidanDelaney

tox.ini

@@ -7,7 +7,7 @@ deps =
      pytest-cov
      flask
      pypy: mock
-commands = pytest -vl --cov=github_webhook --cov-report term-missing --cov-fail-under 100
+commands = pytest -vl --cov=github_webhook --cov-report term-missing --cov-fail-under 98
 
 [testenv:flake8]
 deps = flake8