perf: optimize cast_locale function

[tvuotila]

3x performance boost

get_args_count function is quite slow on Python 3.9.7.

Here are my performance tests:

	non-callable locale	0 argument locale	1 argument locale	2 argument locale
before	1.42 µs ± 82 ns	25.7 µs ± 380 ns	36.6 µs ± 1780 ns	43.7 µs ± 1250 ns
after	1.33 µs ± 93.5 ns	8.9 µs ± 241 ns	5.47 µs ± 286 ns	1.56 µs ± 20.6 ns