fstring

Fixes #413 whereby the NamedTemporaryFile "dest" was out of scope in the
error handling portion of rewrite.

The problem was initially fixed in #414 but it got reverted because of a
linter error. This new commit works around that linter error.

IPython would complain about:

    DeprecationWarning: `magic(...)` is deprecated since IPython 0.13
    (warning added in 8.1), use run_line_magic(magic_name, parameter_s).

…ouble quotes according to PEP257

The error message would previously be confusing. For example, `dotenv -f
. list` would print:

    Error: Invalid value: Path "." does not exist.

Instead, we now print:

    Error opening env file: [Errno 21] Is a directory: '.'

I used this opportunity to slightly refactor the I/O code (e.g. fewer
system calls and possible race conditions) for those two subcommands
(`get` and `list`).

Co-authored-by: Saurabh Kumar <[email protected]>

Thanks @lsmith77

* Fix type hint for load_dotenv

Fixes #431

* Quote type hints to avoid runtime errors in earlier Python versions

* Revise type of dotenv_path parameter

Based on PR feedback and typeshed's type hint for the built-in open()
function:
https://github.com/python/typeshed/blob/e2d67bf7034f68c07bd35150247e58e0817725d9/stdlib/builtins.pyi#L1421

* Allow only string paths, not byte paths

These paths can flow into `shutil.move`, which does not accept byte
paths or (int) file descriptors.  See python/typeshed#6832

* Create a type alias for the paths this library accepts

And use it consistently in main.py.