Integrate mypy into development process (part of #116)

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This PR integrates mypy as a static type checker into the development environment and the CI pipeline. To do that, it fixes a bunch of typing issues in the library code. It solves a part of #116 (but not all of it as that would be too big of a scope).

List of general changes:

• Add mypy to dev environment (tox, Makefile)
• Add mypy to CI pipeline (GitHub Actions)
• Set mypy rules to strict (apart from no_implicit_reexport)

List of code changes:

• Add a lot of missing type hints and fix some incorrect/inaccurate ones
• Replace some type checks with more type-safe ones (e.g. isinstance instead of type(...) is)
• Exception classes: use relative imports to import base ValidationError
• Improve dynamic keyword argument construction in some classes like here
• FloatToDecimalValidator: min_value/max_value will not be cast to string if specified as integer anymore (see commit)
• EnumValidator: Improve type checking of Enum class when creating the validator (9cbf9f9)
• DataclassValidator: (0b6ed6e)
  • Improve check if given dataclass is actually a dataclass
  • Simplified dataclass check and error handling: There is now only one error message for "parameter is a class that is not a dataclass" and "parameter is an instance of a dataclass".
  • Properly type dataclasses.Field with type parameter (requires extra code because in Python 3.8 it is not a Generic yet)
• Use parametrized Pattern type hints for regex patterns (uses typing.Pattern for now, because re.Pattern is not yet parametrized in Python 3.8) (a0d0f54)
• ValidataclassMixin: Add a # type: ignore[call-overload] for the warning about asdict (28110ce)
• Add # type: ignore[override] for validate() overrides in validator classes that return different types than their base validators (e.g. DecimalValidator returning Decimal, which is not compatible with StringValidator returning str) (712ac98)
• Add # type: ignore for sentinel object creation (3965efe)
  • There might be a better way to do this, but sentinels are weird, there should be language support for it.
• unset_to_none(): Use isinstance(value, UnsetValueType) instead of is UnsetValue (again, sentinels are weird) (5ceea74)
• Fix typing in DateTimeRange and DateTimeOffsetRange (f7dd402)
• Various small changes to fix mypy issues