Plan to drop Python 2 support

[sanjayankur31]

Now that Python 2 is no longer supported, we should also start looking into dropping support for it. The process would be something like this:

• pick a release + date that will drop Python 2 support (6 months in the future should be enough?)
• announce this now (well in advance) to community on all public channels
• document what (if anything) users must do to move away from Python 2
• work on updating all our code bases to drop Python 2 support

[sanjayankur31]

Mailed the list, slack and gitter channels today to ask if anyone is still using Python 2 with NeuroML.

[sanjayankur31]

See also #174

[sanjayankur31]

We will drop Python 2 support after the next few releases and announce to the community. Closing this now.