Option for unlimited number of extensions for a Redlock instance.

[btemplep]

Is your feature request related to a problem? Please describe.
There is a case I have when using a Redlock, where I would like to have an unlimited number of extensions.
You can currently set the maximum number of extensions when creating a Redlock with the num_extensions,
but this cannot be set to unlimited.

Describe the solution you'd like
Following the other conventions, setting num_extensions to -1 for unlimited would work.

my_lock = Redlock(
    key="my_key",
    masters={my_master},
    num_extensions=-1 # Allow unlimited calls to my_lock.extend()
)

Describe alternatives you've considered
You can set an arbitrarily large int for num_extensions but it's not as clean.

Additional context
N/A