(#28) Ensure sigterm is handled by coverage

[imranariffin]

This way, coverage can collect data before process termination

Issue was coverage does not handle sigterm by default, so it couldn't get to collect data upon termination: nedbat/coveragepy#1307 (comment)

Need to explicitly tell coverage to handle sigterm: https://coverage.readthedocs.io/en/latest/changes.html#version-6-4-2022-05-22

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: +4.58 🎉

Comparison is base (5c368d7) 94.59% compared to head (f46bf4b) 99.18%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #63      +/-   ##
==========================================
+ Coverage   94.59%   99.18%   +4.58%     
==========================================
  Files          17       17              
  Lines         555      613      +58     
==========================================
+ Hits          525      608      +83     
+ Misses         30        5      -25     

Impacted Files	Coverage Δ
src/aiotaskq/__main__.py	85.00% <ø> (ø)
src/aiotaskq/interfaces.py	100.00% <ø> (ø)
src/tests/test_cli.py	100.00% <ø> (ø)
src/aiotaskq/task.py	98.30% <100.00%> (ø)
src/aiotaskq/worker.py	100.00% <100.00%> (+17.51%)	⬆️
src/tests/apps/simple_app.py	100.00% <100.00%> (+5.00%)	⬆️
src/tests/conftest.py	100.00% <100.00%> (ø)
src/tests/test_concurrency.py	100.00% <100.00%> (ø)
src/tests/test_integration.py	100.00% <100.00%> (ø)

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