Python3.10 Mapping subclass Fix

• Fix added for MutableMapping import using Python 3.10 (#14, thanks @mrkkollo)

Other Changes

• Switched from an ancient Travis config to Github Actions
• Upgrade testing libraries
• Added pyproject.toml
  PRs: #15 , #16

Create paths with set_nested_value(... force=True)

The pelecanus.toolbox module has been added and it includes free-standing versions of the methods that are present inside the PelicanJson object. In addition, the set_nested_value method now includes a force kwarg, which allows users to "force" paths into existence.

Ready for production

First production release: the project is in beta but may be used on nested JSON objects.