biocommons packages dropping support for Python 2.7

[Reece Hart]

All biocommons packages — hgvs, uta, seqrepo, bioutils, and eutils — currently support Python 2.7 and 3.5+. As Python 3 and Python 2 continue to diverge, supporting both language versions has become increasingly difficult and prevents adoption of language features that are available only in Python 3. Therefore, Python 2.7-compatible versions are being phased out and will become obsolete by March 31, 2019. Python 3.6, which was first released nearly two years ago, will be the basis for new development. The expected timeline for this transition is below.

• By October 19, dependencies be pinned to current major.minor versions so that packages that support Python 2.7 will rely only on dependencies that also support Python 2.7. Package development will continue with necessary bug fixes for Python 2.7 and 3.6 until December 31. 
  
• On January 1, support for Python 2.7 will be removed from biocommons packages and package version numbers will be updated to reflect language compatibility. After this date, new packages will not work or be installable in Python 2. Bug fixes and features in Python 3.6 versions will not be backported to 2.7 versions. Although, pull requests for critical bugs in Python 2.7 versions will be accepted, these will generally be low-priority. Some bug fixes may be deemed too difficult or not worth the effort to provide in both versions.
  
• After March 31, no effort will be put into Python 2.7. The most recent Python 2.7-compatible versions will remain available on pypi and in github. Bug reports will be investigated only for Python 3.6 versions of packages.
  

-Reece and Meng