This adds support for pydantic v2 by using the `pydantic.v1` compat module
when pydantic v2 is installed and otherwise falling back to the `pydantic` module.

In order to satisfy mypy, we only import `pydantic.v1` during type
checking. This allows proper type checking without needing to clutter
the code with `type: ignores`.

Once some time is passed, we can entirely drop compatibility for pydantic v1 and
start using the actual pydantic v2 code to reap the additional
performance benefits and other improvements of pydantic v2.

Signed-off-by: Maxwell G <maxwell@gtmx.me>

Old pydantic v1 versions do not define `__version__`.

Signed-off-by: Maxwell G <maxwell@gtmx.me>

Signed-off-by: Maxwell G <maxwell@gtmx.me>

Apparently, towncrier doesn't like this.

Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>