Add an isort configuration (#3463)

setup.cfg

@@ -19,3 +19,15 @@ max-line-length = 90
 #  W503 requires that binary operators be at the end, not start, of lines. Erik doesn't like it.
 #  E203 is contrary to PEP8.
 ignore = W503,E203
+
+[isort]
+line_length = 89
+not_skip = __init__.py
+sections=FUTURE,STDLIB,COMPAT,THIRDPARTY,TWISTED,FIRSTPARTY,TESTS,LOCALFOLDER
+default_section=THIRDPARTY
+known_first_party = synapse
+known_tests=tests
+known_compat = mock,six
+known_twisted=twisted,OpenSSL
+multi_line_output=3
+include_trailing_comma=true