missing setuptools dependency

[Nothing4You]

pynetbox currently depends on setuptools being available, but it does not declare setuptools as runtime dependency:

pynetbox/pynetbox/__init__.py

Line 1 in 944a447

from pkg_resources import get_distribution, DistributionNotFound

pkg_resources is provided by setuptools.

I see 3 ways to solve this:

1. wrap in try block, catching ImportError
2. add explicit setuptools dependency
3. migrate to importlib alternatives as recommended in https://setuptools.pypa.io/en/latest/pkg_resources.html, this can be combined with 1 or 2 (for backported implementations) on older python versions.

[abhi1693]

This will be superseded by #564 when thats merged