Fixing squareroot display issue (#3640)

Calling C.Pow() as was performed prior to commit
4556cfdc26fae8db5adbf10eed72e6305d7aa9a0 to repair issue #3640.
"C.Pow(b, -e)" reintroduced issue #3186, so "evaluate=0" was added
when C.Pow is called to fix #3186 in a different manner:
"C.Pow(b, -e, evaluate=0)".

The following all seem to be correct after this:

>>> sqrt(x)
  ___
╲╱ x
>>> 1 / sqrt(x)
  1
─────
  ___
╲╱ x
>>> pprint (Pow(2,-5,evaluate=0))
1
──
 5
2
>>> print (Pow(2,-5,evaluate=0))
2**(-5)
Merge pull request #1795 from madprime/bugfix/issue_3640

Fixing squareroot display issue (#3640)