* Specify how to install Django 3.2. These tests are currently running
  against later Django versions.
* Add basepython entries for pypy. This fixes running tox locally.
* Fix the cpython version mapping in [gh-actions]. The github action
  tests for cpython versions are running against the latest Django,
  instead of the set of possible Django versions.

There are a few test failures with pypy and Django 3.2. Since 3.2 is out
of long-term support, drop these from the test matrix.

mypy complains when reading or setting a attribute that is not defined
on the class, such as HttpRequest.csp_nonce. This updates the code to
use getattr and setattr to access these dynamically added attributes and
for Django settings.

Both startswith() and parser.parse_statements take a tuple rather than a
list.

Althought the code `template.render(context)` looked similar, mypy
complained that Django's Template could not take a dict. Rather than
switch on types, refactor `make_context` and `make_template` into
`render`, which hides the typing details between Django templates and
extension templates like Jinja2.