Python 2 Deprecated

[vincentqb]

The 0.4.0 release of torchaudio was the last one supporting python 2, and master no longer officially supports python 2. We're looking to strip the code of python 2 references.

• No longer use package six and backports for cross-compatibility
• Convert to inline type hinting
• No __future__ import
• Change string formatting style
• Remove mention of python 2.7 in setup.py
• Remove older code path in _check_module_exists and no longer need to check python 3 is not used at the end of the file
• Update unicode_decoder to python 3 only, here.
• Replace calls to makedir_exist_ok to os.makedirs(.., exist_ok=True)

[tomassosorio]

I can take care of type hinting tomorrow :)

[james-mchugh]

Do you need an extra hand with this or is everything covered?