Bug#762775: pynfft: FTBFS: dh: unable to load addon sphinxdoc

[Aaron M. Ucko]

Source: pynfft
Version: 1.3.2-1
Severity: serious
Justification: fails to build from source

Builds of pynfft in minimal environments geared for building its
architecture-dependent binary packages (e.g., on the autobuilders)
have been failing:

  dh clean --with python2,python3,sphinxdoc --buildsystem=pybuild
  dh: 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/«arch»/perl/5.20.0 /usr/local/share/perl/5.20.0 /usr/lib/«arch»/perl5/5.20 /usr/share/perl5 /usr/lib/«arch»/perl/5.20 /usr/share/perl/5.20 /usr/local/lib/site_perl .) at (eval 13) line 2.
  BEGIN failed--compilation aborted at (eval 13) line 2.
  
  make: *** [clean] Error 2
  debian/rules:10: recipe for target 'clean' failed

Could you please either conditionalize the usage of --with sphinxdoc
appropriately or move python-sphinx into the main Build-Depends field?
In the latter case, please bump the version to (>= 1.2.2+dfsg-2~) to
avoid running into errors if there is no actual documentation to
install.  (See #745690.)

Thanks!