Replace imp by importlib library

[amaltaro]

Impact of the new feature
WMCore in general

Is your feature request related to a problem? Please describe.
While deploying a new agent, I saw this warning message:

/data/srv/wmagent/v2.1.6/sw/slc7_amd64_gcc630/cms/wmagentpy3/2.1.6/bin/wmagent-mod-config:14: DeprecationWarning: the imp module is deprecated in favour of importlib; see the module's documentation for alternative uses
  import imp

Most of the WMCore codebase has already adopted importlib package, but there are still a few leftovers, mainly related to scripts and tests.

Describe the solution you'd like
Search for all usage of the imp library and replace it by importlib.

Describe alternatives you've considered
None

Additional context
Issue seen during WMAgent 2.1.6 deployment.