python-dotenv: update to v0.21.0

Added:

 - CLI: add support for invocations via 'python -m'.
 - load_dotenv function now returns False.
 - CLI: add --format= option to list command.

Fixed:

 - Drop Python 3.5 and 3.6 and upgrade GA
 - Use open instead of io.open.
 - Improve documentation for variables without a value
 - Add parse_it to Related Projects
 - Update README.md
 - Improve documentation with direct use of MkDocs

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