using those included in virtualenv 16.0.0 as the baseline.

Since pip 10.0.0 has dropped support of Python 2.6 and we plan to
keep it until PyCharm 2019.1, now we need to bundle older compatible
versions of all related packages as we previously did for Python <= 2.5.

(cherry picked from commit bff4ba25)

(cherry picked from commit c11f8dd7)