fix needed for python 3.9 (I think)

[knodalyte]

I apologize for writing an issue rather that a pull request, it's a one line fix... Error is:

File "/home/cba/.local/lib/python3.9/site-packages/whichfile/main.py", line 955
repr(s) for s in self.symlink_to,
^
SyntaxError: Generator expression must be parenthesized

[cjwelborn]

Sorry it took so long for a response. I'll see what I can do. Thank you for the report.