Skip zipp.Path when it is not a file

pypy3 has a file-descriptor leak with open('directory') which zipp.Path(...) triggers. This preempts passing a directory and tickling that.

More information:

• https://bitbucket.org/pypy/pypy/issues/3021/ioopen-directory-leaks-a-file-descriptor
• https://github.com/nedbat/coveragepy/issues/807
• https://github.com/pytest-dev/pytest/issues/5342