According to POSIX specification the pathname of user shell is stored in SHELL (environmental variable, see http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html#tag_08) so I think that is good idea to use that pathname instead of hardcoded one.
Change is only in one line of subprocess package to use SHELL environmental variable and use hardcoded shell pathname as fallback.

lines 1431-1433
```
            if shell:
                args = ["/bin/sh", "-c"] + args
                if executable:
```

to

```
            if shell:
                args = [os.environ.get("SHELL","/bin/sh"), "-c"] + args
                if executable:
```