fixed tox.ini parameter passenv

[cehbrecht]

Overview

This PR updates the tox.ini to fix an issue with the passenv parameter.

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

[Zeitsperre]

@cehbrecht Looks good to me.

The only addition I would suggest is to specify tox>=4.0 in the dev dependencies and the main.yml workflow.