Python 3 Compatibility

[tankbusta]

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
.......................................
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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

[fancycode]

Thanks, I slightly modified your changes to keep support for older versions of Python.