Support Django 3.2

[adamchainz]

• Add to both tox and travis grids. This included completing the grid of all Python versions versus Django versions for older Django versions. I moved the GISSERVER_USE_DB_RENDERING tests to only Python 3.9. I think these would be better embedded in the normal test suite with @override_settings to run the few tests that depend on the setting being False rather than whole extra test runs.
• Add the extra DEFAULT_AUTO_FIELD setting for Django 3.2.
• Add changelog note and PyPI classifier.

[vdboor]

Wonderful, thanks for taking care! I wonder, any clue why the Travis builds have failed?

[adamchainz]

They failed because I missed replacing "3.8" for "3.9", so they tried to run the 3.9 tests in an environment where only 3.8 was installed.

[codecov-io]

Codecov Report

Merging #5 (4aef4fb) into master (6941943) will increase coverage by 0.09%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master       #5      +/-   ##
==========================================
+ Coverage   83.60%   83.69%   +0.09%     
==========================================
  Files          36       36              
  Lines        3300     3300              
  Branches      471      471              
==========================================
+ Hits         2759     2762       +3     
+ Misses        373      369       -4     
- Partials      168      169       +1     

Impacted Files	Coverage Δ
gisserver/features.py	77.87% <0.00%> (+1.27%)	:arrow_up:

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