[rrdtool-maint] Bug#942639: fix build with Python3.8 + unusual packaging of the debug extension

[Jean-Michel Vourgère]

Hello Matthias

Thank for your bug report and patch.

Actually, I've been putting the rrdtool*[0-9]dm-*.so file in the general old 
school rrdtool-dbg package. I'm still using non automatic debug symbols, 
because of that damned file. I'll switch to automatic debug symbols at the 
same time. :)

You suggested I use python-rrdtool-dbg, but I suppose you mean python3-
rrdtool-dbg, right? I am unsure of naming convention, and the archive seems to 
have an ugly mix of names.