Python 3 Compatibility

This PR addresses the following:

* Updates the assertions throughout the test suite to remove deprecated ones
* Updates the test suite to use context managers to ensure the test files are closed when the test is done.
* Fixed Python3 compatibility issues in py7zlib

Python 3.4.1 Test Results: 
```
tankbusta in ~/code/third_party/pylzma on fix_unittests λ python3 -m unittest discover
.......................................
----------------------------------------------------------------------
Ran 39 tests in 5.573s

OK
```

Python 2.7.8 Test Results:
```
tankbusta in ~/code/third_party/pylzma on fix_unittests λ python2 -m unittest discover
........................................
----------------------------------------------------------------------
Ran 40 tests in 5.315s

OK
```