Added django-environ-2 because this project is developing independently from joke2k's django-environ.

This notably prevents the file `.nx_file` from being created and not
removed after running tests.  That file could also lead to confusing
test failures when changing and testing the code of python-dotenv.

This is a big change.  It will make it possible to simplify the code,
add more features, improve the robustness and lower the barrier to new
contributions.

As per [Python's packaging documentation][doc], the `python_requires`
keyword argument needs `setuptools >= 24.2.0` (released in 2016) and
will only have en effect for `pip >= 9.0.0` (released in 2016 as well).

[doc]: https://packaging.python.org/guides/distributing-packages-using-setuptools/#python-requires

`to_env` and `to_text` are no longer necessary since they were identity
functions with Python 3.

Now that we only support Python 3, we know that the encoding of source
files is UTF-8 by default: https://www.python.org/dev/peps/pep-3120/.

Since we now require Python 3.5+, we can assume that the `typing` module
is available.  The guards can also be removed because importing that
module is cheap.  This simplifies the code.

Unfortunately, we can't do the same for variables as that is only
supported in Python 3.6+.

Those are synonyms in Python 3.

Mypy would complain about the missing `types-mock` package, which it now
needs to perform accurate type checking and despite
`ignore_missing_imports` set to `True`:

    tests/test_ipython.py:3: error: Library stubs not installed for "mock" (or incompatible with Python 3.9)
    tests/test_ipython.py:3: note: Hint: "python3 -m pip install types-mock"
    tests/test_ipython.py:3: note: (or run "mypy --install-types" to install all missing stub packages)
    tests/test_ipython.py:3: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
    tests/test_main.py:7: error: Library stubs not installed for "mock" (or incompatible with Python 3.9)
    Found 2 errors in 2 files (checked 15 source files)

`set_key` and `unset_key` were more restrictive than other functions
such as `dotenv_values` with regards to their `dotenv_path` argument.

This applies to the `load_dotenv` and `dotenv_values` functions.  This
makes it possible to pass a file stream such as `open("foo", "r")` to
these functions.