[CFT] deskutils/calibre python 3 support

[Guido Falsi]

Hello!

I have committed just now changes to the calibre port to add an option
to build it against python 3. (r528772)

As you all know python 2 is on it's way out and all software must be
migrated, or removed from the ports tree.

You may also know that calibre porting to python 3 is still
experimental, that's why I added an option and did not simply switch
over right away.

At present the port default is still to build with python 2, so nothing
changes by default.

My plan is to switch the default in head after 2020Q2 quarterly is branched.

I'm writing this to ask anyone who uses calibre and has time/resources
to spare to test switching the option to python 3 and test it. This will
become even easier once we branch 2020Q2, since by switching the default
the latest binary packages will get python 3 calibre by default.

I've been running this setup for the last two weeks without problems,
but I only use basic functionality, and I'd especially like the USB
device sync functionality to be stressed a little to make sure nothing
breaks.

Any feedback is appreciated.

Thanks in advance!

-- 
Guido Falsi <madpilot at freebsd.org>