Fix DeprecationWarning about imp module

[leezu]

In 2013, Python Issue #18192 introduced importlib.util.MAGIC_NUMBER and documented deprecated imp.get_magic() as deprecated

With Python 3.7, DeprecationWarnings are shown again by default.

[leezu]

Note this is only available starting Python 3.4. So this should not be merged before January 1, 2019.

[stuartarchibald]

Thanks for the PR. I've patched this further so that Pythons >= 3.1 use importlib and added a test.

[codecov-io]

Codecov Report

Merging #3237 into master will decrease coverage by 0.02%.
The diff coverage is 60%.

@@            Coverage Diff             @@
##           master    #3237      +/-   ##
==========================================
- Coverage    81.1%   81.07%   -0.03%     
==========================================
  Files         386      386              
  Lines       75721    75884     +163     
  Branches     8491     8522      +31     
==========================================
+ Hits        61411    61525     +114     
- Misses      13009    13051      +42     
- Partials     1301     1308       +7

[seibert]

Looks good. Merging.