fix: remove dependency on pkg_resources, moving to implicit namespaces

[JordiSalaJuarez]

pkg_resources.declare_namespace is deprecated, going for the native/implicit namespaces (PEP 420) is a much better option according to setuptools

Fixes issue #140

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[conventional-commit-lint-gcf]

🤖 I detect that the PR title and the commit message differ and there's only one commit. To use the PR title for the commit history, you can use Github's automerge feature with squashing, or use automerge label. Good luck human!

-- conventional-commit-lint bot
https://conventionalcommits.org/

[aqeelat]

@clundin25 can we do the same thing we did in googleapis/google-auth-library-python#1205 here?

[clundin25]

@parthea PTAL

[aqeelat]

@parthea @clundin25 any updates?

[parthea]

Please make a change similar to the one in googleapis/google-auth-library-python#1205

[parthea]

Fixed via #187