FTBFS building without python2

blinker (and maybe other packages) are failing to transition in focal due to the nose package's dependence on python2.

nose provides both py2 and py3 binary packages, however simply dropping the py2 binary package and switching to python3 dependencies is insufficient, because nose's upstream code includes some py2-isms, such as using the 'print' statement instead of the 'print()' function.

See https://launchpad.net/~bryce/+archive/ubuntu/nose-drop-python2/+packages

From `debuild -uc -us -i`:

python3 setup.py build_sphinx
running build_sphinx
Running Sphinx v1.8.5

Configuration error:
There is a syntax error in your configuration file: invalid syntax (core.py, line 153)
Did you change the syntax from 2.x to 3.x?
make[1]: *** [debian/rules:13: override_dh_auto_build] Error 1
make[1]: Leaving directory '/home/bryce/ubuntu/Nose/nose-gu'
make: *** [debian/rules:9: build] Error 2
dpkg-buildpackage: error: debian/rules build subprocess returned exit status 2
debuild: fatal error at line 1182:
dpkg-buildpackage -us -uc -ui -i -I.bzr -I.svn -I.git -i failed

triage-focal+20.04:~/ubuntu/Nose/nose-gu$ grep -n 'print ' nose/*.py
nose/core.py:153: print "%s version %s" % (os.path.basename(sys.argv[0]), __version__)
nose/core.py:227: print "Plugin %s" % p.name
nose/core.py:229: print " score: %s" % p.score
nose/core.py:230: print '\n'.join(textwrap.wrap(p.help().strip(),
nose/core.py:238: print " Options:"
nose/core.py:240: print ' %s' % (', '.join(opts))
nose/core.py:242: print '\n'.join(
nose/inspector.py:205: # print line[end[1]:]
nose/loader.py:324: # print "Check %s (%s) in %s" % (item, test, module.__name__)
nose/proxy.py:93: # name as the actual exception to make it print correctly.
nose/result.py:101: """Overrides to print all errorClasses errors as well.
nose/result.py:113: """Called by the test runner to print the final summary of test
nose/util.py:593: >>> from pprint import pprint

I don't know if there are other python2->3 problems besides the print statement, but typically there's more than that to do...