Dropping Python 2 support

[robpkelly]

Python 2.7 will reach end-of-life on January 1st, 2020 (see this melodramatic countdown timer). Additionally, many major Python projects have pledged to drop support for Python 2 on or before that date.

The next major release of pyGSTi (v0.9.9) will drop support for Python 2. In other words, we'll be limiting our future support to Python 3.5 and 3.7. Users running pyGSTi on Python 2 should consult the official guide for more information on porting their environment to Python 3.

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As of ace6d65 our CI no longer builds for Python 2.7. Developers are no longer required to ensure Python 2.7 compatibility in new contributions. Here's what should be done before v0.9.9:

• Drop Python 2.7 builds from TravisCI
• Specify python_requires in setup.py
• Remove Python 2 backports from dependencies in setup.py
  • Not sure we actually have any strictly for Python 2...
• Remove Python 2 compatibility duct-tape throughout the source tree, including:
  • sys.version_info checks for python 2
  • most if not all __future__ imports
  • Python 2 compatibility patches in pygsti.tools.compattools
• Remove Python 2 static test fixture resources

[robpkelly]

This is all just housekeeping, by the way, so consider this low-priority.

[LSaldyt]

Finally! Best of luck :)

[robpkelly]

#102 closes this, but I'm keeping this issue open until v0.9.9 just to keep users/developers aware