Integrate lxml patch from 4.4 branch

[cehbrecht]

Overview

This PR integrates the lxml patch from the 4.4 branch

Related Issue / Discussion

616

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.
• [x ] I have already previously agreed to the PyWPS Contributions and Licensing Guidelines

[coveralls]

Coverage remained the same at 0.0% when pulling 897308742cfabd9954198476fc80ac0ab8e7fb93 on cehbrecht:integrate-lxml-patch into ec6f6b5f1d6aadc54829cf69a6111113ea2183c6 on geopython:main.

[cehbrecht]

I'm merging this PR. It is the same as PR #616 ... only adapted to the 4.5.x code base.