Replace kaptan with direct PyYAML SafeLoader usage

[rjocoleman]

This PR replaces the kaptan configuration parser with direct usage of PyYAML's SafeLoader class.

Why:

• Kaptan depends on PyYAML>=3.13,<6, which is lower than the currently released 6.0. This can lead to dependency resolution issues.
• Kaptan is pretty much unmaintained with the last release to pypi in 2019 and the only developer who had been committing was unable to get access and deprecated it in their own projects: Admin privileges? emre/kaptan#176

How:

• Yaml file is read if needed, then passed as a string to PyYAML SafeLoader (this is what Kaptan is for this projects usage: https://github.com/emre/kaptan/blob/master/kaptan/handlers/yaml_handler.py#L21)
• PyYAML is added to this project as a dependency instead of kaptan

Caveats:

• Only .yaml and yml file extensions are supported. This check can be easily removed to revert to previous filename behaviour.
• Technically the previous usage of Kaptan allowed yaml, json, pyfile, dict and ini handlers. This was not tested nor documented. If is an actual feature and needs to be maintained then this code will not suffice, and a more feature config reader would be needed (with many more tests!).
• I was not able to get all the tests to pass in my local for master branch, the same tests fail for this branch for me. I don't they are related to my changes.

[legalsylvain]

@sbidoul could you run test ? (As it is a first contribution)

[ap-wtioit]

Tested locally in doodba with python 3.10 and this fixes #72

[sbidoul]

Thanks for this contrib. Since there was a merge conflict I finished it in #76.