Python imp module is now deprecated

[jdtournier]

According to the documentation for the imp module, it is now deprecated:

Deprecated since version 3.4, will be removed in version 3.12: The imp module is deprecated in favor of importlib.

This is used in the mrtrix3.py module to locate and load the main mrtrix3 module. Already causing trouble on the forum. We need to fix this asap...

[Lestropie]

Looks to me like importlib does not have the requisite functionality in Python 2, which is still supported on master. So need to choose between:

• Embed logic in bin/mrtrix3.py to use importlib in Python3 and imp in Python2
• Drop Python2 support on master and tag 3.0.5.

Can change separately on dev since Python2 support has already been dropped there (#2215 pending check in #2534).

[jdtournier]

I reckon we'll need to support both versions for some time. I've had a crack at doing this in #2735, and it's too ugly. I would vastly prefer it if they didn't keep changing things, but there we are...

I'll close this for now, we can discuss on #2735