Update pod component to use generic Iterable and Mapping types

[LewisGaul]

Follows #595, working on #584.

[LewisGaul]

I notice I actually used Sequence in some of the other methods... I'm now questioning whether Iterable or Sequence (or something else?) is preferably here, especially considering a string itself is a valid Iterable[str]...

The "strings are iterables of strings" issue is tracked at python/typing#256. The pondering about Iterable vs. Sequence isn't affected though, since strings are also sequences of strings. I think Iterable should be preferable, to allow the user to pass generators (e.g. docker.volume.inspect((x for x in volume_names if x.startswith("foo"))).

Full list of abstract types: https://docs.python.org/3/library/collections.abc.html#collections-abstract-base-classes

[gabrieldemarmiesse]

Looks good! Thanks for the patch!