Make pip3_cmd a class method.

* modules/pip/lib/puppet/provider/package/pip3.rb(pip3_cmd): The
instances class method was attempting to call the pip3_cmd instance
method, which does not work. Instead define a self.pip3_cmd method
it can call and then delegate the pip3_cmd instance method to that.
Also corrects an error message typo in passing.

Change-Id: I8a7b7ce353ac4293c2d7b82999402fa47dde6051
Reviewed-on: https://review.openstack.org/35708
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins