MAINT Use importlib + packaging instead of pkg_resources

[jeremiedbb]

Fixes #550

the goal is to get rid of this

# TODO we should probably deal with those
'ignore:pkg_resources\ is\ deprecated\ as\ an\ API:DeprecationWarning',
'ignore:Deprecated\ call\ to\ `pkg_resources:DeprecationWarning',

usage of pkg_resources is deprecated. There's still no way to use only importlib to do the same thing, at least without having to manually parse the requirements. This PR uses packaging instead that provides tools to parse the requirements.

Like pkg_resources, packaging is not in the std lib, but is very likely to be installed since it comes with pip. And at least this one is not deprecated :)

[glemaitre]

LGTM.

[LilianBoulard]

Couple of minor changes (which I'll commit) and we're good to merge, thanks!