SA14: Use importlib_metadata<4 to retain compatibility with Python 3.6

Croaks otherwise on `Python 3.6, SA 1.4.36`::

    AttributeError: module 'typing' has no attribute '_SpecialForm'

Author: amotl

setup.py

@@ -67,7 +67,9 @@ def read(path):
         test=['zope.testing>=4,<5',
               'zc.customdoctests>=1.0.1,<2',
               'stopit>=1.1.2,<2'],
-        sqlalchemy=['sqlalchemy>=1.0,<1.5', 'geojson>=2.5.0']
+        sqlalchemy=['sqlalchemy>=1.0,<1.5',
+                    'geojson>=2.5.0',
+                    'importlib_metadata<4']
     ),
     python_requires='>=3.4',
     install_requires=requirements,