Static analysis of type hints using `mypy`

[marvinpfoertner]

Is your feature request related to a problem? Please describe.
probnum uses type hints quite extensively. However, since we do not run any static analysis tool on them, they are very prone to errors.

Describe the solution you'd like.

• add mypy to linting-requirements.txt and pin the major version number
• add a mypy environment to the tox configuration (similar to the pylint environment)
• add mypy configuration to the pyproject.toml file
• add a mypy check to our GitHub Actions CI pipeline, initially only checking
  • probnum._function
  • probnum.pn_method

Additional context
None