The check to see whether the passed-in mode parameter was a valid member

of the enum was incorrect (at least until Python 3.12), due to a

leftover from the implementation before the mode was implemented as an

enum. This was causing a TypeError inside __contains__, since that

can't check for values, only instances.
This change updates it to attempt to construct a Mode enum from the

value, and catching the error if it's not a correct value.

Verified that new try/except was the correct way to handle this.
Ran unit tests.