Increase minimum Requests version to 2.20

[cjmayo]

Add pdfminer.six to minimum version testing

[mgedmin]

What's with the docs failures?

WARNING: [autosummary] failed to import linkcheck.checker.
Possible hints:
* AttributeError: module 'lib' has no attribute 'X509_V_FLAG_CB_ISSUER_CHECK'
* ImportError: 
* AttributeError: module 'linkcheck' has no attribute 'checker'

[cjmayo]

docs uses requirements.txt, adding pdfminer.six here then meant it pulled in cryptography, but cryptography >= 38 breaks pyOpenSSL < 22 by removing that X509_V_FLAG_CB_ISSUER_CHECK: pyca/pyopenssl@d290855 and Ubuntu 20.04 only has 19.0.0 (the other jobs get pyOpenSSL upgraded by tox.ini because it is used directly in the tests).

ubuntu-latest was meant to have been updated to 22.04 a few days ago, https://github.blog/changelog/2022-12-01-github-actions-larger-runners-using-ubuntu-latest-label-will-now-use-ubuntu-22-04/. But not seeing that here, so I have changed the docs job to explicitly use ubuntu-22.04.

Couple of other changes:

• I had originally set the minimum pdfminer.six to 20181108 but changed it to 20191020 while experimenting with the docs problem. Put it back to 20181108. That is the same as Debian oldstable.
• Added pdfminer.six to the list of packages in install.txt for a manual setup.