Modernize CI using Tox, add a "[dev]" recipe, re-enable coveralls

[Zeitsperre]

Overview

This PR does a bit of cleanup to the package's CI and current tests.

• Updates the tox.ini to reflect the current state of the package.
• Adds a [dev] recipe to the setup.py so that developers can install pywps for development with pip install -e (pywps/.)[dev].
• Removes a few warning raised from the test ensemble by specifying that raw strings are being used with regex.
• Update the GitHub CI workflow to use tox, re-enabling the Coveralls check (the last successful run was uploaded in July 2020).
• Updates the License to include $CURRENT_YEAR

Related Issue / Discussion

Additional Information

Contribution Agreement

(as per https://github.com/geopython/pywps/blob/master/CONTRIBUTING.rst#contributions-and-licensing)

• I'd like to contribute [feature X|bugfix Y|docs|something else] to PyWPS. I confirm that my contributions to PyWPS will be compatible with the PyWPS license guidelines at the time of contribution.
• I have already previously agreed to the PyWPS Contributions and Licensing Guidelines

[coveralls]

Coverage increased (+81.1%) to 81.137% when pulling 8f2fcca on tox_modernisation into c6a0d7e on main.