Remove future dependency

[alanyee]

Using a thirdparty dependency as big as future for just urlencode() does not seem worth it

[adamchainz]

Please consider this PR. future triggers a warning on Python 3.8:

  /.../future/standard_library/__init__.py:65: DeprecationWarning: the imp module is deprecated in favour of importlib; see the module's documentation for alternative uses
    import imp

intuit-oauth is the only package in the project I'm looking at that requires future.

It would also make sense to remove Python 2 compatibility altogether, since it was EOL nearly 2 years ago.