Bunch of testing and README updates

[briantist]

SUMMARY

Somewhat related: #81

With two releases of hvac broken on Py2 (inadvertently), I've started thinking it's time to drop Python2 support, hence the linked issue.

This PR started as trying to fix the tests to ensure they don't use the affected versions of hvac, some other changes went in.

Testing

• constraints.txt and requirements.txt updated to stop at hvac 0.10.11 in Python 2. As of this writing, 0.10.14 hasn't been released yet but it's expected it will restore Py2 compatibility. At that time I may change the constraints to exclude the two affected, and let it bring in newer ones, we'll see.
• The custom playbook for installing requirements in integration tests was not using the constraints file included in the collection. That has been changed.
• The requirements playbook's lines for choosing very old versions of hvac to work with old versions of OSes that are no longer tested against, have been removed.

CI

• Reduced the number of Vault versions being tested against from 3 down to 2, and updated which versions to keep in line with newer Vault releases. The strain on CI was getting higher and it seems GitHub has reduced the available concurrency so CI runs were taking ever longer, especially considering the previous addition of Ansible 2.11 to the matrix.
• Added Python 3.10 testing, only in devel and only in the latest Vault version for now, in accordance with: Changes impacting Collection Contributors and maintainers overview#45 (comment)

Documentation

README updated to:

• describe hvac requirements, add Py2 notice and link to issue
• better describe supported python versions
• better describe supported Ansible versions

ISSUE TYPE

COMPONENT NAME

ADDITIONAL INFORMATION

[codecov]

Codecov Report

Merging #82 (e792740) into main (08f5270) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #82   +/-   ##
=======================================
  Coverage   82.88%   82.88%           
=======================================
  Files          16       16           
  Lines         783      783           
  Branches       78       78           
=======================================
  Hits          649      649           
  Misses        120      120           
  Partials       14       14           

---

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 08f5270...e792740. Read the comment docs.