add pytest-cov

[nsano-rururu]

$ MS_TEAMS_WEBHOOK=MicrosoftWebhookURL
$ export MS_TEAMS_WEBHOOK
$ pytest --cov=./pymsteams --cov-report=term-missing --cov-branch
==================================================================================== test session starts =====================================================================================
platform linux -- Python 3.7.10, pytest-5.0.1, py-1.10.0, pluggy-0.12.0
rootdir: /home/sano/dkwork3/pymsteams
plugins: cov-3.0.0
collected 8 items                                                                                                                                                                            

test/test_webhook.py ........                                                                                                                                                          [100%]

---------- coverage: platform linux, python 3.7.10-final-0 -----------
Name                    Stmts   Miss Branch BrPart  Cover   Missing
-------------------------------------------------------------------
pymsteams/__init__.py     148     21     42     11    82%   52->54, 56->58, 66-74, 77-78, 81-82, 94->96, 116->118, 124, 143, 184-188, 191->194, 205-206, 225-226, 241, 254, 257, 259->exit, 264-265
-------------------------------------------------------------------
TOTAL                     148     21     42     11    82%

================================================================================== 8 passed in 2.95 seconds ==================================================================================