py-ipython: updated to 7.15.0

IPython 7.15 brings a number of bug fixes and user facing improvements.

Misc Noticeable changes:
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- Long completion name have better elision in terminal
- I've started to test on Python 3.9 and fix some errors.
- Hi DPI scaling of figures when using qt eventloop
- Document the ability to have systemwide configuration for IPython.
- Fix issues with input autoformatting
- ``IPython.core.debugger.Pdb`` is now interruptible
- Video HTML attributes

(adam)