drop support for Python 3.6

Author: dimitri-yatsenko

datajoint/table.py

@@ -311,7 +311,7 @@ def update1(self, row):
             raise DataJointError("Update cannot be applied to a restricted table.")
         key = {k: row[k] for k in self.primary_key}
         if len(self & key) != 1:
-            raise DataJointError("Update entry must exist.")
+            raise DataJointError("Update can only be applied to one existing entry.")
         # UPDATE query
         row = [
             self.__make_placeholder(k, v)

setup.py

@@ -3,7 +3,7 @@
 from os import path
 import sys
 
-min_py_version = (3, 6)
+min_py_version = (3, 7)
 
 if sys.version_info <  min_py_version:
     sys.exit('DataJoint is only supported for Python {}.{} or higher'.format(*min_py_version))