ImportError: cannot import name 'gcd' from 'fractions' (/usr/lib64/python3.9/fractions.py)


In Python 3.9 , "gcd" is deprecated : https://docs.python.org/3.5/library/fractions.html#fractions.gcd

Fix : 

```
$ diff commpy/channelcoding/gfields.py*
6c6
< #from fractions import gcd
---
> from fractions import gcd
112c112
<             orders[idx] = (2**self.m - 1)/(math.gcd(i, 2**self.m-1))
---
>             orders[idx] = (2**self.m - 1)/(gcd(i, 2**self.m-1))
```