Drop Python 2.7 support

[franasa]

Since January 1, 2020, there is no official support for Python 2.7 anymore, meaning that no more bug fixes or security updates will be released. I couldn't find any discussion about this, so I'm opening this one to ask whether a complete switch to python 3.6+ is planned for the python library, or if there are any reasons not to do it?

[javh]

No objections from me. Though, I think we should wait until AIRR v2 before dropping Python 2 support, because it's a major change.

[schristley]

It's going to take some time to update workflows (on VDJServer) that use python2. The AIRR v2 timeframe seems reasonable to me.

[javh]

From the call: Agreed to convert to Python 3 in v2.

[schristley]

While I don't have VDJServer completely converted yet, it's going quicker than the AIRR V1.4 release, so we might want to consider dropping for V1.4 instead.

[javh]

All right, swapped the milestone. We can decide once we approach v1.4. Sound good?

[javh]

From the call:

• We can drop python support in v1.4 to enable the Germline schema.