Peter, please include x.y.z version with reports (and upgrade first if possible ;-).  This was apparently fixed in #25532 last year for current versions.  With 3.6.4 and 3.7.0b2 I get
Traceback (most recent call last):
  File "F:\Python\a\tem2.py", line 3, in <module>
    print(inspect.unwrap(unittest.mock.call))
  File "C:\Programs\Python37\lib\inspect.py", line 515, in unwrap
    raise ValueError('wrapper loop when unwrapping {!r}'.format(f))
ValueError: wrapper loop when unwrapping call