Although these are synonyms in Python 2.x, when using MyPy to typecheck
code use of file() causes spurious errors.
This commit also serves as one small step to Python 3.x compatibility,
since 3.x removes the file() builtin.
e.g. assert x == y -> self.assertEqual(x, y);
self.assertTrue(isinstance(x, y)) -> self.assertIsInstance(x, y)
These specific methods give more useful errors in the case of a test
failure.
On my laptop (Ubuntu 17.10, Python 2.7.14 in a virtualenv),
`test_regular_mod` fails with
```
AssertionError: "\nimport sys\n\n\ndef say_hi():\n print 'hi'\n" !=
'\x03\xf3\r\n\xbbW\xd5Yc\x00\x00\x00\x00\x00\x00\x00\x00\x02\x00\x00\x00@\x00\x00\x00s\x19\x00\x00\x00d\x00\x00d\x01\x00l\x00\x00Z\x00\x00d\x02\x00\x84\x00\x00Z\x01\x00d\x01\x00S(\x03\x00\x00\x00i\xff\xff\xff\xffNc\x00\x00\x00\x00\x00\x00\x00\x00\x01\x00\x00\x00C\x00\x00\x00s\t\x00\x00\x00d\x01\x00GHd\x00\x00S(\x02\x00\x00\x00Nt\x02\x00\x00\x00hi(\x00\x00\x00\x00(\x00\x00\x00\x00(\x00\x00\x00\x00(\x00\x00\x00\x00sF\x00\x00\x00/home/alex/src/mitogen/tests/data/module_finder_testmod/regular_mod.pyt\x06\x00\x00\x00say_hi\x05\x00\x00\x00s\x02\x00\x00\x00\x00\x01(\x02\x00\x00\x00t\x03\x00\x00\x00sysR\x01\x00\x00\x00(\x00\x00\x00\x00(\x00\x00\x00\x00(\x00\x00\x00\x00sF\x00\x00\x00/home/alex/src/mitogen/tests/data/module_finder_testmod/regular_mod.pyt\x08\x00\x00\x00<module>\x02\x00\x00\x00s\x02\x00\x00\x00\x0c\x03'
```
`__file__` contains the path of the compiled `.pyc`, not the `.py`
source file.
Can't figure out what it's supposed to do any more, and can't find a
version of Ansible before August 2016 (when I wrote that code) that
seems to need it.
Add some more mitigations to avoid sending dylibs.
Alex Willmer