Issue 164, pipdeptree doesn't work with python 3.11

[dqkqd]

Hi, I'm trying to fix issue #164 which makes pipdeptree does not work with python 3.11.

Since python 3.11, pip try to use importlib.metadata.Distribution instead of pkg_resources.Distribution.
https://github.com/pypa/pip/blob/e2c9c40edeb65b9d26c13a12dbe5642dc2e00fd0/src/pip/_internal/metadata/__init__.py#L45-L49

Currently there is a workaround which is set _PIP_USE_IMPORTLIB_METADATA=0 as mentioned in #164 (comment).
But I think it should be better to explicit import pkg_resources and use it.

[gaborbernat]

Please rebase on main branch.

[gaborbernat]

This PR adds 1k lines and remove 1k. In this form is just too much. We'll need to reopen a new PR that's cleanly on top of the current main.

[dqkqd]

Sorry for the mess with git, I created a new one #173