Investigate why test_create_in_symlink test broke after upgrading to virtualenv 20

[tjanez]

Figure out why the test_create_in_symlink() test:
https://github.com/berdario/pew/blob/24a4b7d6fa760d34a7348686c1ce1fd5e72847ba/tests/test_mkvirtualenv.py#L20-L27

broke after upgrading to virtualenv 20 in #214.

Support for virtualenv creation in symlinked directories was originally added in #145.