Update travis config and formatting docs

[tolstislon]

• Travis CI added mypy, flake8, and black checks
• Update formatting doc string

[codecov]

Codecov Report

Merging #28 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master       #28   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            6         6           
  Lines          337       337           
=========================================
  Hits           337       337           

Impacted Files	Coverage Δ
testrail_api/_category.py	100.00% <ø> (ø)
testrail_api/_session.py	100.00% <ø> (ø)

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