Remove imports/adjustments to make NAV run on Python<3.9/Django<3.2

[johannaengland]

Another part to #2790. Closes #2839.

Improvements due to changes in Python 3.8/3.9 will come in a separate PR.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 56.36%. Comparing base (b8d6d52) to head (f1bab55).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2903 +/- ## ======================================= Coverage 56.35% 56.36% ======================================= Files 603 601 -2 Lines 43862 43827 -35 Branches 48 48 ======================================= - Hits 24719 24702 -17 + Misses 19131 19113 -18 Partials 12 12 ```

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Test results

       9 files         9 suites   7m 33s :stopwatch: 3 320 tests 3 320 :heavy_check_mark: 0 :zzz: 0 :x: 6 379 runs  6 379 :heavy_check_mark: 0 :zzz: 0 :x:

Results for commit 5f650591.

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