Add initial tests

[major]

This PR adds initial tests for sktm and uses pytest to run them. We can run the tests with plain unittest later, but pytest gives us some nice features we could use.

• Requires Db: Remove patchsource_id FOREIGN KEY #64 to merge first to fix the foreign key error

[major]

The coverage report is a bit ugly since there is no test coverage from previous commits. ;)

[danghai]

Hi @major , it is good to use coverage the same as skt use. What do you think?

[major]

@danghai I was using coveralls, but their bot that posts feedback in the PR has been causing issues lately.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@eeeda05). Click here to learn what that means.
The diff coverage is 100%.

@@           Coverage Diff            @@
##             master     #61   +/-   ##
========================================
  Coverage          ?   23.6%           
========================================
  Files             ?       5           
  Lines             ?     915           
  Branches          ?       0           
========================================
  Hits              ?     216           
  Misses            ?     699           
  Partials          ?       0

Impacted Files	Coverage Δ
tests/test_init.py	100% <100%> (ø)

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[dzickusrh]

this looks fine as a start. I filed issue #70 to track small changes later.