Reach 100% test coverage

[florimondmanca]

Closes #96

• Applies relevant pragma: no cover invocations
• Add missing admin and permission tests.
• Test migrations using the fantastic django-test-migrations package. (This was created in Nov 2019. Not available at the time of version 1.4, which contained #62 that added the 0004 migration. Would have been so helpful to ensure migrations work in various use cases!)

[codecov-commenter]

Codecov Report

Merging #211 (9abbc11) into master (ead1b3e) will increase coverage by 3.05%. The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           master      #211      +/-   ##
===========================================
+ Coverage   96.94%   100.00%   +3.05%     
===========================================
  Files          22        23       +1     
  Lines         523       579      +56     
===========================================
+ Hits          507       579      +72     
+ Misses         16         0      -16     

Impacted Files	Coverage Δ
src/rest_framework_api_key/__init__.py	100.00% <ø> (+20.00%)	:arrow_up:
tests/test_compatibility.py	100.00% <ø> (+11.11%)	:arrow_up:
tests/test_admin.py	100.00% <100.00%> (+2.43%)	:arrow_up:
tests/test_migrations.py	100.00% <100.00%> (ø)
tests/test_permissions.py	100.00% <100.00%> (+0.99%)	:arrow_up:
src/rest_framework_api_key/permissions.py	100.00% <0.00%> (+5.26%)	:arrow_up:
src/rest_framework_api_key/admin.py	100.00% <0.00%> (+23.07%)	:arrow_up:
...ework_api_key/migrations/0004_prefix_hashed_key.py	100.00% <0.00%> (+28.57%)	:arrow_up:

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