LocalPath implementations and documentation for test mocking

[jayqi]

• New abstract subclasses LocalPath and LocalClient that use the local filesystem as "cloud" storage.
• New classes LocalAzureBlobClient, LocalAzureBlobPath, LocalS3Client, LocalS3Path that are intended to be monkeypatch substitutes for testing (UPDATE: +LocalGSClient, LocalGSPath)
• New documentation article "Testing with cloudpathlib" "Testing code that uses cloudpathlib" with examples of how to patch things for tests
• New test rigs that run the full test suite on the new local classes

Resolves #106

[github-actions]

🚀 Deployed on https://deploy-preview-107--gallant-agnesi-5f7bb3.netlify.app

[codecov]

Codecov Report

Merging #107 (423f705) into master (da72bb3) will increase coverage by 0.0%.
The diff coverage is 91.9%.

@@           Coverage Diff           @@
##           master    #107    +/-   ##
=======================================
  Coverage    91.5%   91.5%            
=======================================
  Files          11      18     +7     
  Lines         839    1034   +195     
=======================================
+ Hits          768     947   +179     
- Misses         71      87    +16     

Impacted Files	Coverage Δ
cloudpathlib/local/implementations/azure.py	84.8% <84.8%> (ø)
cloudpathlib/local/implementations/gs.py	86.6% <86.6%> (ø)
cloudpathlib/local/implementations/s3.py	86.6% <86.6%> (ø)
cloudpathlib/local/localpath.py	94.4% <94.4%> (ø)
cloudpathlib/local/localclient.py	97.3% <97.3%> (ø)
cloudpathlib/cloudpath.py	90.1% <100.0%> (ø)
cloudpathlib/local/__init__.py	100.0% <100.0%> (ø)
cloudpathlib/local/implementations/__init__.py	100.0% <100.0%> (ø)
cloudpathlib/s3/s3path.py	97.8% <100.0%> (ø)
... and 6 more

[jayqi]

@pjbull I think this is ready for review!

[jayqi]

@pjbull I rebased this PR and added support for the new GS classes.

[pjbull]

🎉

Would be good to add a bullet to "why cloudpathlib" at the top of the readme that says something like "Testability: We provide Local* classes that can be used to easily mock cloud storage in your unit tests."