bpo-40955: Fix memory leak in subprocess module (GH-20825)

```
Direct leak of 8 byte(s) in 1 object(s) allocated from:
    #0 0x7f008bf19667 in __interceptor_malloc (/lib64/libasan.so.6+0xb0667)
    #1 0x7f007a0bee4a in subprocess_fork_exec /home/heimes/dev/python/cpython/Modules/_posixsubprocess.c:774
    #2 0xe0305b in cfunction_call Objects/methodobject.c:546
```

Signed-off-by: Christian Heimes <christian@python.org>

Misc/NEWS.d/next/Library/2020-06-12-11-55-30.bpo-40955.huixCg.rst

@@ -0,0 +1 @@
+Fix a minor memory leak in :mod:`subprocess` module when extra_groups was specified.

Modules/_posixsubprocess.c

@@ -893,6 +893,7 @@ subprocess_fork_exec(PyObject* self, PyObject *args)
     if (_enable_gc(need_to_reenable_gc, gc_module)) {
         pid = -1;
     }
+    PyMem_RawFree(groups);
     Py_XDECREF(preexec_fn_args_tuple);
     Py_XDECREF(gc_module);