small fixes for use with python 3.4, postgres, and many-to-many relations

[denys-duchier]

replaced file->open (file is no longer a builtin in python3.4). also postgres is very grumpy about integrity constraint violations (as it should be): it stopped complaining after these fixes. Ah yes... also: table names need to be quoted if e.g. they have the bad taste of using camelcase (which unfortunately one of my apps did).

[jwhitlock]

Interesting bucket of issues.

• The same py3 fix was added in 4c8ce36
• The table quoting fix looks interesting - I'd like to see covering tests, but it may require Add tests for FastFixtureTestCase #220
• The m2m code is functionally identical to Find tables of many-to-many relationships #214. The bug appears to be FastFixtureTestCase._fixture_teardown ignores m2m relations in fixtures #109.

FastFixtureTestCase fixes will probably come after REUSE_DB=1 fixes, but if you want to write some tests (#220) and submit individual PRs, it might get fast tracked.