Port to Python3

Brief summary of the modifications:

* Use six for compatibility with both Python 2 and 3;
* Replace UserDict.DictMixin with collections.MutableMapping;
* Fix relative imports;
* Use test-requirements.txt for requirements that are common to both Python 2
  and 3, and test-requirements-py{2,3}.txt for version-specific requirements;
* Miscellaneous fixes.
* Use a specific test_db_py3.cfg file for Python 3, that only runs tests on
  sqlite.

Thanks to Victor Stinner who co-wrote this patch.

Change-Id: Ia6dc536c39d274924c21fd5bb619e8e5721e04c4
Co-Authored-By: Victor Stinner <victor.stinner@enovance.com>

1 files changed, 4 insertions, 3 deletions

diff --git a/migrate/changeset/ansisql.py b/migrate/changeset/ansisql.py
index b4509ae..a18d4ed 100644
--- a/migrate/changeset/ansisql.py
+++ b/migrate/changeset/ansisql.py
@@ -4,7 +4,6 @@
    At the moment, this isn't so much based off of ANSI as much as
    things that just happen to work with multiple databases.
 """
-import StringIO
 
 import sqlalchemy as sa
 from sqlalchemy.schema import SchemaVisitor
@@ -20,6 +19,7 @@ from migrate import exceptions
 import sqlalchemy.sql.compiler
 from migrate.changeset import constraint
 from migrate.changeset import util
+from six.moves import StringIO
 
 from sqlalchemy.schema import AddConstraint, DropConstraint
 from sqlalchemy.sql.compiler import DDLCompiler
@@ -43,11 +43,12 @@ class AlterTableVisitor(SchemaVisitor):
         try:
             return self.connection.execute(self.buffer.getvalue())
         finally:
-            self.buffer.truncate(0)
+            self.buffer.seek(0)
+            self.buffer.truncate()
 
     def __init__(self, dialect, connection, **kw):
         self.connection = connection
-        self.buffer = StringIO.StringIO()
+        self.buffer = StringIO()
         self.preparer = dialect.identifier_preparer
         self.dialect = dialect