fix: remove dependency on pkg_resources, moving to implicit namespaces

[jsaladev]

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

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[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