Fix warnings and minor cleanup

[c00kiemon5ter]

I noticed several warnings while testing and trying the code. This is a cleanup for all but one remaining warning which is part of defusedxml:

pysaml2/venv/lib/python3.7/site-packages/defusedxml/ElementTree.py:68:
DeprecationWarning: The html argument of XMLParser() is deprecated
  _XMLParser.__init__(self, html, target, encoding)

The changeset also removes some code that refers to RSA type of Cryptodome.PublicKey which is no longer used.

All Submissions:

• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Have you added an explanation of what problem you are trying to solve with this PR?
• Have you added information on what your changes do and why you chose this as your solution?
• Have you written new tests for your changes?
• Does your submission pass tests?
• This project follows PEP8 style guide. Have you run your code against the 'flake8' linter?

[codecov-io]

Codecov Report

Merging #520 into master will increase coverage by 0.04%.
The diff coverage is 60.86%.

@@            Coverage Diff             @@
##           master     #520      +/-   ##
==========================================
+ Coverage   65.17%   65.21%   +0.04%     
==========================================
  Files         100      100              
  Lines       25693    25660      -33     
==========================================
- Hits        16746    16735      -11     
+ Misses       8947     8925      -22

Impacted Files	Coverage Δ
src/saml2/sigver.py	70.81% <ø> (+1.51%)	⬆️
src/saml2/server.py	72.6% <ø> (ø)	⬆️
src/saml2/config.py	84.85% <ø> (ø)	⬆️
src/saml2/mongo_store.py	72.2% <ø> (-0.2%)	⬇️
src/saml2/s2repoze/plugins/formswithhidden.py	0% <0%> (ø)	⬆️
src/saml2/mdbcache.py	62.1% <100%> (ø)	⬆️
src/saml2/__init__.py	86.33% <100%> (-0.17%)	⬇️
src/saml2/eptid.py	96.07% <50%> (-3.93%)	⬇️
src/saml2/cert.py	94.52% <60%> (+3.32%)	⬆️
src/saml2/saml.py	92.28% <62.5%> (+0.2%)	⬆️
... and 4 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f06a327...47391d4. Read the comment docs.