Fix Code.getargs() treating co_flags bitmask values as counts (#14493)

CO_VARARGS (4) and CO_VARKEYWORDS (8) are bitmask flags, but they were
used directly as increment values for the argument count via `&`.
This caused co_varnames[:argcount] to slice beyond the actual arguments
when the function body contained enough local variables.

Fix by wrapping the bitmask results in bool(), so they contribute
at most 1 to the argcount.

Closes #14492