mythtv and python not installable in testing/unstable

Hi,

With the python2 removal, python-requests-cache as been removed from
Debian testing/unstable and thus mythtv isn't installable.

Python3 support isn't ready and should be in mythtv 31

https://github.com/MythTV/mythtv/pull/182#issuecomment-536176585

https://www.mythtv.org/wiki/Python3_Conversion

Christian