devel/ipython: Update to 6.5.0

• devel/ipython is now tracking 6.x mainline
• Previous version of ipython (5.x series) moved to devel/ipython5
• It is based on pure Python 3.3+ codebase
• Supports latest Python 3.7
• Remove GCC dependency since it now builds with CLANG

More information about new features as well as backward incompatible changes [1]

[1] https://ipython.readthedocs.io/en/stable/whatsnew/version6.html

Reviewed by: koobs, mat
Approved by: koobs (python)
Differential Revision: https://reviews.freebsd.org/D16508