Fix resource leak in WheelFile.open()

[jdufresne]

In WheelFile.open(), if the hash does not exist, avoid opening the file
before raising the exception. Previously would leak the open file
resource which could result in a ResourceWarning when Python warnings
are enabled:

ResourceWarning: unclosed file <_io.FileIO name='…/test_testzip_missing_hash0/test-1.0-py2.py3-none-any.whl' mode='rb' closefd=True>

Python warnings are now enabled during tests to help catch these
earlier.

[agronholm]

I will merge this if you rebase the code against latest master.

[jdufresne]

Thanks. Rebased.

[agronholm]

Thanks!