Adom Lock with Contextmanager

[icovada]

This PR adds a couple extras like removing unused imports and a dangerous default "{}" in a function call

Breaking changes:

• login() and logout() return None, not a Session()
• login() raises ValueError if login fails

It looks enormous because of the formatting I've run on it.

[icovada]

Added commits 946394 and e108e3a with ContextManagers.

The goal is to avoid leaving open sessions and/or stale ADM locks should your code have an issue between open and close, you can call this library like so:

with FortiManager([data]) as fm:
    with Lock([data]) as admlock:
        fm.do_stuff()

instead of

fm = FortiManager()
fm.login()
fm.lock_adm("whatever")
fm.do_stuff()
fm.unlock_adm("whatever")
fm.logout()

[icovada]

Done.
Final code should be:

with FortiManager([data]) as fm:
    adm_lock = fm.get_lock()  # can pass an ADOM name to get a lock for that ADOM
    with adm_lock:
        do_stuff()

[moustic999]

Hello, It seems there is a missing "return self" in the enter method of FortiManager. This make the with statement giving None Object !

[icovada]

Oops... well this PR is already merged, I'd need a new one, can't you fix this yourself?