Right now, loading a PEFT config saved with a more recent PEFT version
than is currently installed will lead to errors when new arguments are
added to the config in the newer PEFT version. The current workaround is
for users to manually edit the adapter_config.json to remove those
entries.

With this PR, PEFT will make an attempt at removing these unknown keys
by inspecting the signature. The user will be warned about these removed
keys. This should generally be a safe measure because we will generally
not introduce new config settings that change the default behavior.
However, if a non-default is used, this could lead to wrong results.
This is mentioned in the warning.

While working on the tests, I also converted the unittest.TestCase to a
normal pytest test in order to be able to use pytest fixtures.

I also plan on adding the PEFT version to the adapter_config.json in the
future. This will allow us to better handle compatibility issues in the
future. As adding that new key to all PEFT configs could cause a lot of
disruption, I want to get this PR in first to ensure forward
compatibility.

Note that this new mechanism will not help anyone using a PEFT version
<= 0.12.0, so this will be a slow transition.