Use psycopg2cffi module instead of psycopg2 if running on pypy

The psycopg2 package isn't available for PyPy.  This commit adds a check
if the runtime is PyPy, and if it is uses psycopg2cffi module in favor
of psycopg2. This is almost a drop-in replacement, except for one place
where an additional cast to string is required.

synapse/storage/_base.py

@@ -376,7 +376,7 @@ class SQLBaseStore(object):
         Returns:
             A list of dicts where the key is the column header.
         """
-        col_headers = list(intern(column[0]) for column in cursor.description)
+        col_headers = list(intern(str(column[0])) for column in cursor.description)
         results = list(
             dict(zip(col_headers, row)) for row in cursor
         )

synapse/storage/engines/__init__.py

@@ -18,6 +18,7 @@ from .postgres import PostgresEngine
 from .sqlite3 import Sqlite3Engine
 
 import importlib
+import platform
 
 
 SUPPORTED_MODULE = {
@@ -31,7 +32,12 @@ def create_engine(database_config):
     engine_class = SUPPORTED_MODULE.get(name, None)
 
     if engine_class:
-        module = importlib.import_module(name)
+        needs_pypy_hack = (name == "psycopg2" and
+                platform.python_implementation() == "PyPy")
+        if needs_pypy_hack:
+            module = importlib.import_module("psycopg2cffi")
+        else:
+            module = importlib.import_module(name)
         return engine_class(module, database_config)
 
     raise RuntimeError(