Version 0.5.5

[lossyrob]

Update version to 0.5.5 in preparation for release

PR Checklist:

• Code is formatted (run scripts/format)
• Tests pass (run scripts/test)
• This PR maintains or improves overall codebase code coverage.
• Changes are added to the CHANGELOG. See the docs for information about adding to the changelog.

[codecov-io]

Codecov Report

Merging #278 (8f61062) into develop (3bc1bf2) will decrease coverage by 0.37%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #278      +/-   ##
===========================================
- Coverage    94.05%   93.68%   -0.38%     
===========================================
  Files           32       33       +1     
  Lines         3956     4055      +99     
===========================================
+ Hits          3721     3799      +78     
- Misses         235      256      +21     

Impacted Files	Coverage Δ
pystac/version.py	100.00% <100.00%> (ø)
pystac/__init__.py	100.00% <0.00%> (ø)
pystac/extensions/__init__.py	100.00% <0.00%> (ø)
pystac/extensions/file.py	78.35% <0.00%> (ø)

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