master fails to build/check

[Greg Troxel]

I see the following from scons check.  I don't remember exactly when I
last did this, but it was likely within the week.

(As an aside, it seems irregular for this sort of compile stuff to be
happening at check time vs build time, but perhaps this is check-only code.)

  Creating 'zerk'
  chmod -w zerk
  chmod +x zerk
  cp gpscap.py tmp.py; /usr/pkg/bin/python2.7 -tt -m py_compile tmp.py;
  rm -f tmp.py*
  cp gpssim.py tmp.py; /usr/pkg/bin/python2.7 -tt -m py_compile tmp.py;
  rm -f tmp.py*
  cp jsongen.py tmp.py; /usr/pkg/bin/python2.7 -tt -m py_compile tmp.py;
  rm -f tmp.py*
  cp maskaudit.py tmp.py; /usr/pkg/bin/python2.7 -tt -m py_compile tmp.py;
  rm -f tmp.py*
  cp test_clienthelpers.py tmp.py; /usr/pkg/bin/python2.7 -tt -m py_compile 
tmp.py;
  rm -f tmp.py*
  cp test_misc.py tmp.py; /usr/pkg/bin/python2.7 -tt -m py_compile tmp.py;
  rm -f tmp.py*
  cp test_xgps_deps.py tmp.py; /usr/pkg/bin/python2.7 -tt -m py_compile tmp.py;
  rm -f tmp.py*
  cp valgrind-audit.py tmp.py; /usr/pkg/bin/python2.7 -tt -m py_compile tmp.py;
  rm -f tmp.py*
  cp example_aiogps.py tmp.py; /usr/pkg/bin/python2.7 -tt -m py_compile tmp.py;
    File "tmp.py", line 23
      async def get_gps_updates(gpsd: gps.aiogps.aiogps) -> None:
              ^
  SyntaxError: invalid syntax

  scons: *** [python-compilation-regress] Error 1
  scons: building terminated because of errors.
  WARNING: asciidoctor not found.
  WARNING: Some documentation and html will not be built.
  WARNING: ncurses not found, not building cgps or gpsmon.


This is odd, because I set PYTHON=python3.7 and call build as

  scons target_python=${PYTHON} ${SCONS_ARGS} > OUT.10_build 2>&1 

I expected this to be cached and used for check, but maybe not, and thus
just amended by check call to

  scons target_python=${PYTHON} check ${SCONS_ARGS} > OUT.20_check 2>&1

and still the same result.

I am unclear on if async def is a python3-ism, but guessing so.  It
seems like it's a bug for the build system to have found 2.7 and to be
using it to compile a python3-only file.  Especially when I passed in
target_python explicitly.  (My system has 2.7, 3.7 and 3.8 installed,
which seems relatively normal.)

Any ideas?