prepare for 0.4.0 release

[philvarner]

Related Issue(s):

n/a

Description:

0.4.0 release

PR Checklist:

• Code is formatted
• Tests pass
• Changes are added to the CHANGELOG

[codecov-commenter]

Codecov Report

Merging #235 (76198d5) into main (6841066) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #235   +/-   ##
=======================================
  Coverage   83.40%   83.40%           
=======================================
  Files          10       10           
  Lines         711      711           
=======================================
  Hits          593      593           
  Misses        118      118           

Impacted Files	Coverage Δ
pystac_client/version.py	100.00% <100.00%> (ø)

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