Use pure Python code for scanf

[olebole]

The sscanf module is the only module that needs to be compiled on macOS, and this module requires to have a C compiler installed (or to build wheels) to install PyRAF there. To simplify this, we switch to pure Python for the scanf module.

The module is taken from joshburnett/scanf and modified to behave similarly to the sscanf C module. The only major difference is the missing "n" format switch, which gives the number of characters read in so far. Not sure if this is ever needed.

Compared with the CL scanf documentation, the new Python module is compliant (except for the h modifier, which was also not in the old C module):

A field format specification has the form "%[][W][lh]C", where '' indicates the field should be skipped, W is the field width, 'l' indicates longword output, 'h' indicates halfword output, and C is the format code. The format codes C are as follows:

c    single character (c or '\c' or '\0nnn')
d    decimal integer
e    exponential format
f    fixed format
g    general format
o    octal integer
s    string
x    hexadecimal integer