Skosprovider-atramhasis

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- Upgrade requirements. Including skosprovider 1.1.0 (116)
- Retry GET calls within _request method 5 times before giving up (120)

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- The variabele scheme_id in AtramhasisProvider is used before the variabele is defined. (106)

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- Upgrade libraries, fe dogpile.cache (95)
- Drop support for python 2 (96)
- Only build property concept_scheme once (97)
- Lazy access concept_scheme.uri (98)

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- Fix a bug in `dict_to_thing` that resulted in the provider exposing objects
instead of id's in relations. (80)

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- Update to `Skosprovider 0.7.0 <https://pypi.org/project/skosprovider/0.7.0/>`_.
- Add caching support to the provider. (75)
- Don't load concepscheme at startup, since this prevents the provider from
instantiating when an Atramhasis instance is down. (66)
- This is the last version to support Python 2.7. Updated Python 3 support to
3.6, 3.7 and 3.8.

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- Update supported Python version (61)
- Add a long_description_content_type (55)
- Correct handling of get_by_uri (69)