Deprecate Use of imp in Favor of importlib on Python >= 3.1

[derks]

The imp module is deprecated in favor of importlib as of Python 3.4 ... should update Cement (with backward compat for imp on Python < 3.1).

[akhilman]

I have a problem with importlib.

We have a structure:

./test_imp.py
./test_importlib.py
./plug/foo/__init__.py
./plug/foo/bar.py

./plug/foo/__init__.py

from .bar import baz

./plug/foo/bar.py

baz = 1

./test_imp.py

import imp

# NOTE we do not need to specify file path and all that suffixes
f, path, desc = imp.find_module('foo', ['./plug'])
mod = imp.load_module('foo', f, path, desc)
print(mod.baz)

./test_importlib.py

import importlib
import importlib.util

spec = importlib.util.spec_from_file_location('foo', './plug/foo/__init__.py')
mod = importlib.util.module_from_spec(spec)
spec.loader.exec_module(mod)
print(mod.baz)

imp example works well but importlib raises with

Traceback (most recent call last):
  File "test_miportlib.py", line 6, in <module>
    spec.loader.exec_module(mod)
  File "<frozen importlib._bootstrap_external>", line 665, in exec_module
  File "<frozen importlib._bootstrap>", line 222, in _call_with_frames_removed
  File "./plug/foo/__init__.py", line 1, in <module>
    from .bar import baz
SystemError: Parent module 'foo' not loaded, cannot perform relative import

This might be related to https://bugs.python.org/issue18018
I propose to use imp until we get relative import work.