Link.to_dict() should only contain strings

[jsignell]

Related Issue(s):
I know this issue is closed, but I think this PR does what the OP intended.

• link.py to_dict is not properly serializing RelType enum values #652

Description:

I just encountered this recently when checking how a dict roundtrips. This is what I was doing:

import pystac
import requests

response = requests.get("https://staging-stac.delta-backend.com/collections/no2-monthly-diff")
collection_dict = response.json()
collection_dict["links"]

[{'rel': 'items',
  'type': 'application/geo+json',
  'href': 'https://staging-stac.delta-backend.com/collections/no2-monthly-diff/items'},
 {'rel': 'parent',
  'type': 'application/json',
  'href': 'https://staging-stac.delta-backend.com/'},
 {'rel': 'root',
  'type': 'application/json',
  'href': 'https://staging-stac.delta-backend.com/'},
 {'rel': 'self',
  'type': 'application/json',
  'href': 'https://staging-stac.delta-backend.com/collections/no2-monthly-diff'}]

collection = pystac.Collection.from_dict(collection_dict)
collection.validate()
collection.to_dict()["links"]

Before this PR

[{'rel': 'items',
  'href': 'https://staging-stac.delta-backend.com/collections/no2-monthly-diff/items',
  'type': 'application/geo+json'},
 {'rel': 'parent',
  'href': 'https://staging-stac.delta-backend.com/',
  'type': 'application/json'},
 {'rel': <RelType.ROOT: 'root'>,
  'href': 'https://staging-stac.delta-backend.com/',
  'type': <MediaType.JSON: 'application/json'>,
  'title': 'veda-stac'},
 {'rel': 'self',
  'href': 'https://staging-stac.delta-backend.com/collections/no2-monthly-diff',
  'type': 'application/json'}]

After this PR

[{'rel': 'items',
  'href': 'https://staging-stac.delta-backend.com/collections/no2-monthly-diff/items',
  'type': 'application/geo+json'},
 {'rel': 'parent',
  'href': 'https://staging-stac.delta-backend.com/',
  'type': 'application/json'},
 {'rel': 'root',
  'href': 'https://staging-stac.delta-backend.com/',
  'type': 'application/json',
  'title': 'veda-stac'},
 {'rel': 'self',
  'href': 'https://staging-stac.delta-backend.com/collections/no2-monthly-diff',
  'type': 'application/json'}]

PR Checklist:

• pre-commit hooks pass locally
• Tests pass (run scripts/test)
• Documentation has been updated to reflect changes, if applicable
• 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-commenter]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (70efbb6) 90.33% compared to head (79036b4) 90.33%.

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Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1114   +/-   ##
=======================================
  Coverage   90.33%   90.33%           
=======================================
  Files          48       48           
  Lines        6321     6321           
  Branches      941      941           
=======================================
  Hits         5710     5710           
  Misses        429      429           
  Partials      182      182           

Impacted Files	Coverage Δ
pystac/link.py	90.35% <100.00%> (ø)

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