Fix broken build mypy errors

[jayqi]

1. Updates AzureBlobClient._get_metadata return type per discussion in Failed build on master branch (tests #417) #172
2. Explicitly silences mypy errors from azure.storage.blob.*. I decided we should implement this workaround because neither of these two following things seem like they're on track to getting fixed.
   • The Azure SDK typing errors are out of our control, but the upstream issue hasn't had any news in 9 days. Type hint errors in new azure-storage-blob release Azure/azure-sdk-for-python#20771
   • There is some kind of bug in mypy, as site-packages errors are supposed to be ignored but aren't. Specifying to ignore python module in configuration file python/mypy#11239 (comment)

Closes #172

[github-actions]

🚀 Deployed on https://deploy-preview-177--gallant-agnesi-5f7bb3.netlify.app

[codecov]

Codecov Report

Merging #177 (90f3fe4) into master (8e4dcaf) will not change coverage.
The diff coverage is 100.0%.

@@          Coverage Diff           @@
##           master    #177   +/-   ##
======================================
  Coverage    94.1%   94.1%           
======================================
  Files          21      21           
  Lines        1189    1189           
======================================
  Hits         1120    1120           
  Misses         69      69           

Impacted Files	Coverage Δ
cloudpathlib/azure/azblobclient.py	94.6% <100.0%> (ø)

[pjbull]

Thanks @jayqi!