Clarify that plistlib's load and dump functions take a binary file object (GH-9825)

Clarify that plistlib's load and dump functions take a binary file object (GH-9825)

The documentation says that the fp parameter to plistlib.load "should be a
readable and binary file object" but the docstring only mentions that it
should be readable. Similarly, plistlib.dump's docstring only mentions
"writable". This commit clarifies that fp should also be binary.

https://docs.python.org/3/library/plistlib.html#plistlib.load
https://docs.python.org/3/library/plistlib.html#plistlib.dump