Python 3.7 in testing/experimental

[Matthias Klose]

The Python 3.7 beta 5 packages are now in testing, and experimental has 
python3-defaults packages which add 3.7 as a supported version.  The release 
candidate is expected next week, only adding Unicode 11 support, and the final 
release is expected at the end of June.

I would appreciate it, if somebody could run a test rebuild using the 
python3-defaults from experimental.  Else, please test your packages using 
python3.7.

Matthias