Changes

OBS: build_srcpkg.Dockerfile: add sphinx-common

Install sphinx-common, as it is needed to build the source package for
pyosmocom.

Fix for:
  dh clean --with python3,sphinxdoc --buildsystem=pybuild
  dh: error: unable to load addon sphinxdoc: Can't locate Debian/Debhelper/Sequence/sphinxdoc.pm in @INC (you may need to install the Debian::Debhelper::Sequence::sphinxdoc module) (@INC contains: /etc/perl /usr/local/lib/x86_64-linux-gnu/perl/5.36.0 /usr/local/share/perl/5.36.0 /usr/lib/x86_64-linux-gnu/perl5/5.36 /usr/share/perl5 /usr/lib/x86_64-linux-gnu/perl-base /usr/lib/x86_64-linux-gnu/perl/5.36 /usr/share/perl/5.36 /usr/local/lib/site_perl) at (eval 13) line 1.
  BEGIN failed--compilation aborted at (eval 13) line 1.

Change-Id: I598f0b18c44522adc1a038df9dc4d05c7f39e4b1