The value of `quote_mode` must now be one of `auto`, `never` or
`always`, to ensure that users aren't accidentally relying on any other
value for their scripts to work.

Surrounding quotes are no longer stripped.  This makes it possible for
the user to control exactly what goes in the .env file.  Note that when
doing `dotenv set foo 'bar'` in Bash, the shell will have already
removed the quotes.

Single quotes are used instead of double quotes.  This avoids
accidentally having values interpreted by the parser or Bash (e.g. if
you set a password with `dotenv set password 'af$rb0'`.

Previously, the `auto` mode of quoting had the same effect as `always`.
This commit restores the functionality of `auto` by not quoting
alphanumeric values (which don't need quotes).  Plenty of other kinds of
values also don't need quotes but it's hard to know which ones without
actually parsing them, so we just omit quotes for alphanumeric values,
at least for now.

Mypy was failing because the new version requires some type packages to
be installed even when `ignore_missing_imports` is set to `true`.

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.