GitHub workflows

[fopina]

Following up on #49, this PR replaces Travis with Github workflows

Workflows are based on what I regularly use in my own projects (eg, this or this)

Some notes:

• updated test requirements to include pytest instead of nose as .travis.yml and these requirements were inconsistent
  • not sure which one was the "latest" but I personally use pytest so I went for it 😄
• the tests workflow includes a commented job for style checks, same as travis did
  • the issue in comments is resolved by a PR that does not include any flake8 nor black config (in setup.py or a pyproject.toml), so I left it the same way to be address in different PR
• included a workflow to be able to test publishing packages using test.pypi.org (by using a branch named testpypi-*)
  • for these branches, tests will be executed twice if a PR is open
    • it can be solved by slightly modifying tests workflow to not trigger if PR and source branch is testpypi* but I felt like it wasn't worth (running tests is fast and it only happens twice for these special branches WITH open PR)

I hope these helps!

[fopina]

@allburov ready for review!
Just in time to use it to release 1.0.9 😄

Btw, all workflows tested in https://github.com/fopina/dohq_teamcity/actions

[mashinkareva]

LGFM

[allburov]

1.0.9 release is available! https://pypi.org/project/dohq-teamcity/
shout-out to @fopina 🥇