Fix devcontainer setup

[phxnsharp]

• Update Python to 3.9.
• Remove postStartCommand. User will be required to follow all of the commands from the contributing documentation inside the container.

[phxnsharp]

Fixes #67

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.00%. Comparing base (993f2f8) to head (461c725).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #68   +/-   ##
=======================================
  Coverage   80.00%   80.00%           
=======================================
  Files           5        5           
  Lines         235      235           
=======================================
  Hits          188      188           
  Misses         47       47           

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