Always update pip for python3 virtualenvs

[kosmos342]

to resolve the problem when pip 9.0.1(default for ubuntu 18.04) can't install some packages.

Example:

pip 9.0.1:

(virtualenv) jenkins@neko:/var/lib/jenkins/workspaces/ws-template-importer$ pip install pandas -i https://xxx:xxx@yolapi.yola.net/simple/
Collecting pandas
  Downloading https://yolapi.yola.net/media/dists/pandas-1.1.5.tar.gz (5.2MB)
    100% |████████████████████████████████| 5.2MB 286kB/s
    Complete output from command python setup.py egg_info:
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "/tmp/pip-build-vki5a2zj/pandas/setup.py", line 793, in <module>
        setup_package()
      File "/tmp/pip-build-vki5a2zj/pandas/setup.py", line 763, in setup_package
        ext_modules=maybe_cythonize(extensions, compiler_directives=directives),
      File "/tmp/pip-build-vki5a2zj/pandas/setup.py", line 520, in maybe_cythonize
        raise RuntimeError("Cannot cythonize without Cython installed.")
    RuntimeError: Cannot cythonize without Cython installed.

    ----------------------------------------
Command "python setup.py egg_info" failed with error code 1 in /tmp/pip-build-vki5a2zj/pandas/

new pip(20.3.2):

(virtualenv) jenkins@neko:/var/lib/jenkins/workspaces/ws-template-importer$ pip install pandas -i https://xxx:xxx@yolapi.yola.net/simple/
....
Successfully installed numpy-1.19.4 pandas-1.1.5 python-dateutil-2.8.1 pytz-2020.1 six-1.15.0