feat: add support for case insensitive env names

[jasonkuhrt]

feat: add support for case insensitive env names

Change Summary

BaseSettings can now be configured to read from os.environ in a
case-insensitive way.

Related issue number

Arguably will close #277. Doesn't quite meet the exact idea as proposed there, however it achieves the end goal that motivated #277, I think.

Performance Changes

pydantic cares about performance, if there's any risk performance changed on this PR,
please run make benchmark-pydantic before and after the change:

• before: pydantic best=32.086μs/iter avg=33.792μs/iter stdev=1.696μs/iter
• after: pydantic best=31.856μs/iter avg=32.579μs/iter stdev=0.731μs/iter

Checklist

• Unit tests for the changes exist
• Tests pass on CI and coverage remains at 100%
• Documentation reflects the changes
• No performance deterioration (if applicable)
• HISTORY.rst has been updated
  • if this is the first change since a release, please add a new section
  • include the issue number or this pull request number #<number>
  • if you're not a regular contributer please include your github username @whatever

[jasonkuhrt]

I will complete the other TODOs once I know the PR has a good chance of being merged.

[codecov]

Codecov Report

Merging #313 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #313   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          13     13           
  Lines        1703   1708    +5     
  Branches      321    323    +2     
=====================================
+ Hits         1703   1708    +5

[samuelcolvin]

Definitely happy to merge once complete.

[jasonkuhrt]

@samuelcolvin thanks, ready for review again.

[jasonkuhrt]

I came to the conclusion that env_val was a better ident name now that we have env_vars above. env_var in context of env_vars sounds like its an item from the list. Buts its not. It is a value from the environ dict while env_vars is said environ dict.

If you prefer to not touch env_var ident name, I would suggest using a different name than env_vars above. Maybe normalized_environ?

[samuelcolvin]

Looks great. Thank you very much.

[jasonkuhrt]

Thanks for the amazingly prompt collaboration! 🙏