Add a new option: use_tls

[loic-vial]

Hello !

Thank you for the work done on this plugin ! I am currently using it for accessing JupyterHub in our data science team at my company, but I needed to add an option "use_tls" and set it to False to actually make it work. That is because we need to connect to a LDAP server that needs this configuration.

For information, such option exists in the LDAP connector used by Airflow, another tool that we are also using. There is distincts "USE_SSL" and "USE_TLS" options for addressing our use case, so I just applied the same logic.

I hope that my PR is correct, I simply added the option "use_tls" and a small test for testing the option. I also needed to update the black pre-commit hook because it didn't work on my computer.

Obviously, the default value of this option does not change anything on the current behavior.

Thank you !

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[consideRatio]

I think this makes sense, it clarifies that actually TLS is used by default as well, but I'm struggling still a bit as there seem to be two ways of handling TLS.

(From googling ldap port tls)

This config seems related to StartTLS rather than encrypted from the start, right? Hmmm... Overall learning about things still and can't decide yet on pressing merge or not.

[consideRatio]

SSL and TLS --- its confusing to say "use_ssl=False" and "use_tls=True" at the same time, and its also confusing to have TLS when use_ssl is false as we do now.

Argh.