Adds metadata attribute for entity_category_support

[skanct]

This pull request adds support for the metadata attribute http://macedir.org/entity-category-support. This is required (at least) for the IdPs expressing support for entity categories like the research and scholarship and code of conduct

Submission checklist:

• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Have you added an explanation of what problem you are trying to solve with this PR?
• Have you added information on what your changes do and why you chose this as your solution?
• Have you written new tests for your changes?
• Does your submission pass tests?
• This project follows PEP8 style guide. Have you run your code against the 'flake8' linter?

[codecov-io]

Codecov Report

Merging #573 into master will decrease coverage by 0.04%.
The diff coverage is 28.57%.

@@            Coverage Diff            @@
##           master    #573      +/-   ##
=========================================
- Coverage   65.64%   65.6%   -0.05%     
=========================================
  Files         103     103              
  Lines       25594   25601       +7     
=========================================
- Hits        16801   16795       -6     
- Misses       8793    8806      +13

Impacted Files	Coverage Δ
src/saml2/config.py	84.52% <100%> (+0.04%)	⬆️
src/saml2/metadata.py	45.17% <16.66%> (-0.34%)	⬇️
src/saml2/validate.py	74.9% <0%> (-3.14%)	⬇️

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