HOTFIX: Added missing requirements file for publishing to PyPI.

Merge pull request #14 from Paebbels/dev

Changes:
* changed test runner to pytest
* test reported issues (code snippets from issue reports)

Fixed some documentation issues.

Moving CI jobs from Travis-CI to GitHub workflows.

More documentation and first ideas for parser testing.

Changed to new RTD yaml format version 2. Doc restructuring.

Updated shields in README and added introduction to the RTD documenta…

…tion.