server default comparison for blank string fails for Postgresql / Py3.7

[zzzeek]

diff --git a/tests/test_postgresql.py b/tests/test_postgresql.py
index 73300f5..da945d8 100644
--- a/tests/test_postgresql.py
+++ b/tests/test_postgresql.py
@@ -506,6 +506,9 @@ class PostgresqlDefaultCompareTest(TestBase):
             None, col, rendered, cols[0]["default"]
         )
 
+    def test_compare_string_blank_default(self):
+        self._compare_default_roundtrip(String(8), '')
+
     def test_compare_interval_str(self):
         # this form shouldn't be used but testing here
         # for compatibility

in py37 we get:

psycopg2.ProgrammingError: unterminated quoted string at or near "'''"
LINE 1: SELECT ''::character varying = '''

in py36 no problem.

[zzzeek]

so it's a broken regex:

import re

rendered_metadata_default = "''"
rendered_metadata_default = re.sub(
    r"^u?'?|'?$", "'", rendered_metadata_default
)

print(rendered_metadata_default)

py36: '' py37: '''

[sqla-tester]

Mike Bayer has proposed a fix for this issue in the master branch:

Clarify and correct PostgreSQL server default comparison for py37 https://gerrit.sqlalchemy.org/1150