Python extensions complain about PYTHONPATH

When I try to run extensions or importers that require Python, I always get a dialog saying:

Your PYTHONPATH points to a site-packages dir for Python 3.x but you are running Python 2.x!
     PYTHONPATH is currently: "/Applications/Inkscape 2.app/Contents/Resources/share/inkscape/extensions:/Applications/Inkscape 2.app/Contents/Resources/lib/python2.7/site-packages/"
     You should `unset PYTHONPATH` to fix this.

However, my PYTHONPATH is being set in the Inkscape launcher file.
When I run Inkscape from the command line with DEBUG set, I see this output:

+ export VERSIONER_PYTHON_VERSION=2.7
+ VERSIONER_PYTHON_VERSION=2.7
+ '[' x86_64 = i386 ']'
+ export VERSIONER_PYTHON_PREFER_32_BIT=no
+ VERSIONER_PYTHON_PREFER_32_BIT=no
++ python -V
++ cut -c 8-10
+ PYTHON_VERS=2.7
+ export PYTHONPATH=/Applications/Inkscape.app/Contents/Resources/lib/python2.7/site-packages/
+ PYTHONPATH=/Applications/Inkscape.app/Contents/Resources/lib/python2.7/site-packages/

I've tried this both with versions
"Inkscape 0.91+devel+osxmenu r12922, built Jun 16 2015, 09:24:00"
and
"Inkscape 0.91 r13725"

I'm running os Mac OS/X 10.12.6; both versions of Inkscape have been working fine for quite a while, and I haven't changed them recently.

PYTHONPATH isn't set in my shell or at the system level.
There is no executable called 'python' other than the system Python 2.7 in /usr/bin .

I do have both Python2.7.14 and Python3.6.4 installed via Homebrew, but its executable is called '/usr/local/bin/python3'. I have tried prepending /usr/bin to my PATH and running from the command line with no difference.

How can I fix this?