fix: random crashes in credentials.py

[markste-in]

os.path.sep and os.sep are supposed to do the same but os.path.sep can lead to random crashes in newer python versions with the following error

AttributeError: module 'posixpath' has no attribute 'sep'

This is also known to other projects and is caused by some broken python versions

pytest-dev/pytest#10547
Toblerity/Fiona#1143
pallets/jinja#1697

The small proposed fix will avoid the issue altogether, even with broken python versions

[nateprewitt]

Thanks for the report, @markste-in. It's not clear for the other issues why this is happening. All the explanation appear vague. I don't think this is an unreasonable change though. I'm going to close/open the issue to get CI running since it appears to have not run when you opened the PR. We'll take a closer look after that finishes.

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.09% 🎉

Comparison is base (97ba590) 93.29% compared to head (bc69c32) 93.39%.
Report is 209 commits behind head on develop.

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Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2946      +/-   ##
===========================================
+ Coverage    93.29%   93.39%   +0.09%     
===========================================
  Files           64       66       +2     
  Lines        13588    13957     +369     
===========================================
+ Hits         12677    13035     +358     
- Misses         911      922      +11     

Files Changed	Coverage Δ
botocore/credentials.py	98.43% <100.00%> (ø)

... and 13 files with indirect coverage changes

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[nateprewitt]

Thanks, @markste-in!