add datetime representer and allow unicode characters

[KatKatKateryna]

Overview

This PR is fixing the behavior of overwriting example-config.yaml via admin API. Currently, the data is received correctly but YAML writer is representing datetime objects and special characters incorrectly:

To fix this, YAML writer now has 'allow_unicode=True' and 'Dumper=MyDumper', where MyDumper is an instance of yaml.SafeDumper with correct datetime object representers.

Related Issue / discussion

This was discovered while working on QGIS pygeoapi-config plugin: correctly sent data is being saved incorrectly on the running pygeoapi instance: byteroad/pygeoapi_config#12

Additional information

Dependency policy (RFC2)

• I have ensured that this PR meets RFC2 requirements

Updates to public demo

• I have ensured that breaking changes to the pygeoapi master demo server have been addressed
  • https://github.com/geopython/demo.pygeoapi.io/blob/master/services/pygeoapi_master/local.config.yml

Contributions and licensing

(as per https://github.com/geopython/pygeoapi/blob/master/CONTRIBUTING.md#contributions-and-licensing)

• I'd like to contribute [feature X|bugfix Y|docs|something else] to pygeoapi. I confirm that my contributions to pygeoapi will be compatible with the pygeoapi license guidelines at the time of contribution
• I have already previously agreed to the pygeoapi Contributions and Licensing Guidelines

[doublebyte1]

Thank you @KatKatKateryna ! 🌟 A couple of things:

• Could you tick the box on "Updates to Public Demo"? I don't think the content of this PR is breaking anything on the demo server 👍🏽
• please run flake8 to fix the formatting of the code: https://docs.pygeoapi.io/en/latest/development.html#using-flake8
• I think this test needs to be adjusted:
  

  pygeoapi/tests/api/test_admin.py

  Line 80 in 5d75055

  def test_admin(monkeypatch, admin_config_path, openapi):

[doublebyte1]

See my comments above, Thanks!

[KatKatKateryna]

@doublebyte1 Thanks! I am not sure about the test, because the only relevant check is for 'put_config' method, but it only checks for the return code. In the end of the method, though, it does call 'admin.write(data)' where the change was made, but the affected function only writes data to YAML and never does any validation from that file. So the only way it would affect the test, is if the yaml writer fails for some reason.