Eric's basic approach sounds fine to me.

The "pre-compiled .pyc files won't trigger SyntaxWarning" problem isn't new, as it exists for the old 3.5 warnings as well (-B prevents writing bytecode, which may be handy while working on this. Unfortunately, there's no equivalent to prevent reading it except deleting the offending bytecode file):

  $ python3 -B -c "import syntax_warning" 
  /home/ncoghlan/devel/py36/syntax_warning.py:3: SyntaxWarning: name 'x' is assigned to before global declaration
    global x
  $ python3 -c "import syntax_warning" 
  /home/ncoghlan/devel/py36/syntax_warning.py:3: SyntaxWarning: name 'x' is assigned to before global declaration
    global x
  $ python3 -c "import syntax_warning" 
  $ rm __pycache__/syntax_warning.cpython-35.pyc 
  $ python3 -B -c "import syntax_warning" 
  /home/ncoghlan/devel/py36/syntax_warning.py:3: SyntaxWarning: name 'x' is assigned to before global declaration
    global x

As long as folks are running their tests at least once in fresh environments they'll see the warning.