PyLong_FromString(): fix Coverity CID 1424951 (#4738)

Explicitly cast digits (Py_ssize_t) to double to fix the following
false-alarm warning from Coverity:

"fsize_z = digits * log_base_BASE[base] + 1;"

CID 1424951: Incorrect expression (UNINTENDED_INTEGER_DIVISION)
Dividing integer expressions "9223372036854775783UL" and "4UL", and
then converting the integer quotient to type "double". Any remainder,
or fractional part of the quotient, is ignored.