Remove future dependency

[cdce8p]

The project already requires Python 3.8 or newer. There is no reason to keep the future dependency any longer.
This will be necessary to provide support for Python 3.12, as future doesn't seem to be updated any longer and the current version will be incompatible with 3.12. PythonCharmers/python-future#246

https://github.com/poljar/matrix-nio/blob/5354a4d5c03b9b85f36f0b8e5c153a6fe5dec8f9/pyproject.toml#L19

venv/lib/python3.11/site-packages/future/standard_library/__init__.py:65
  /.../venv/lib/python3.11/site-packages/future/standard_library/__init__.py:65: DeprecationWarning:
      the imp module is deprecated in favour of importlib and slated for removal in Python 3.12; see the module's documentation for alternative uses
    import imp

Fixes #435