Opened https://github.com/python/cpython/pull/17064 to fix this. Essentially it's a tiny little oversight in the back-porting. In the 3.7 branch, we perform an attribute lookup for `from_param` before the union checking code, so we must remember to DECREF it. This is unlike master where the attribute lookup happens after the union checking code.
λ win32\python_d.exe -m test -R 3:3 test_ctypes
0:00:00 Run tests sequentially
0:00:00 [1/1] test_ctypes
beginning 6 repetitions
123456
......
== Tests result: SUCCESS ==
1 test OK.
Total duration: 14.8 sec
Tests result: SUCCESS