Enforce multiprocessing start method spawn in test suite

[jakob-keller]

Description of Change

Enforce multiprocessing start method spawn in test suite.

Assumptions

Replace this text with any assumptions made (if any)

Checklist for All Submissions

• I have added change info to CHANGES.rst
• If this is resolving an issue (needed so future developers can determine if change is still necessary and under what conditions) (can be provided via link to issue with these details): closes Test suite uses unsafe multiprocessing start method fork on Linux #1274
  • Detailed description of issue
  • Alternative methods considered (if any)
  • How issue is being resolved
  • How issue can be reproduced
• If this is providing a new feature (can be provided via link to issue with these details):
  • Detailed description of new feature
  • Why needed
  • Alternatives methods considered (if any)

Checklist when updating botocore and/or aiohttp versions

• I have read and followed CONTRIBUTING.rst
• I have updated test_patches.py where/if appropriate (also check if no changes necessary)
• I have ensured that the awscli/boto3 versions match the updated botocore version

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.51%. Comparing base (43f8d7c) to head (0e8c280).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1275      +/-   ##
==========================================
- Coverage   87.53%   87.51%   -0.03%     
==========================================
  Files          67       67              
  Lines        5913     5917       +4     
==========================================
+ Hits         5176     5178       +2     
- Misses        737      739       +2     

Flag	Coverage Δ
unittests	87.51% <100.00%> (-0.03%)	⬇️

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