The "avoid rebuilding" part doesn't seem to work for me. Source build currently fails as follows:

"""
/bin/mkdir -p Include
python ./Parser/asdl_c.py -h Include ./Parser/Python.asdl
# Substitution happens here, as the completely-expanded BINDIR
# is not available in configure
sed -e "s,@EXENAME@,.../INSTALL/py3km/bin/python3.5dm," < ./Misc/python-config.in >python-config.py
# Replace makefile compat. variable references with shell script compat. ones;  -> 
sed -e 's,\$(\([A-Za-z0-9_]*\)),\$\{\1\},g' < Misc/python-config.sh >python-config
# On Darwin, always use the python version of the script, the shell
# version doesn't use the compiler customizations that are provided
# in python (_osx_support.py).
if test `uname -s` = Darwin; then \
		cp python-config.py python-config; \
	fi
Traceback (most recent call last):
  File "./Parser/asdl_c.py", line 1312, in <module>
    main(args[0], dump_module)
  File "./Parser/asdl_c.py", line 1251, in main
    if not asdl.check(mod):
  File ".../cpython/Parser/asdl.py", line 183, in check
    v = Check()
  File ".../cpython/Parser/asdl.py", line 140, in __init__
    super().__init__()
TypeError: super() takes at least 1 argument (0 given)
"""

Python installation is 2.7 (originally 2.5 at the system level, but recent build changes broke that), no Py3 available.