python-dotenv: update to v0.19.0

Changed:

 - Require Python 3.5 or a later version. Python 2 and 3.4 are no
 longer supported

 - Raise ValueError if quote_mode isn't one of always, auto or never
 in set_key

 - When writing a value to a .env file with set_key or dotenv set
 <key> <value>

Added:

 - The dotenv_path argument of set_key and unset_key now has a type of
 Union[str, os.PathLike] instead of just os.PathLike

Signed-off-by: Javier Marcet <javier@marcet.info>