There seems to be a performance issue when creating a Mock() object from unittest module.
The performance difference between 3.7.x and 3.8.0 is about 7-8 times slower in 3.8

Heres the smalles sample i could generate:

Using python 3.7.5

```
python3 -m timeit -v --number=100000 --setup="from unittest.mock import Mock" "Mock()"
raw times: 2.99 sec, 2.96 sec, 3.33 sec, 2.98 sec, 2.92 sec

100000 loops, best of 5: 29.2 usec per loop
```


Using python 3.8.0
```
python3 -m timeit -v --number=100000 --setup="from unittest.mock import Mock" "Mock()"
raw times: 16.9 sec, 17 sec, 17.7 sec, 18.1 sec, 16.3 sec

100000 loops, best of 5: 163 usec per loop

```

I did not find that issue, but a co-worker.