Upgrade pip to fix `sphinx` install error

$ dev/create-release/do-release-docker.sh -d /tmp/spark-2.4.4 -n

$ docker build -t spark-rm-test2 --build-arg UID=501 dev/create-release/spark-rm

root@4e196b3d7611:/# lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 16.04.6 LTS
Release:        16.04
Codename:       xenial

root@4e196b3d7611:/# pip install sphinx
Collecting sphinx
  Downloading https://files.pythonhosted.org/packages/89/1e/64c77163706556b647f99d67b42fced9d39ae6b1b86673965a2cd28037b5/Sphinx-2.1.2.tar.gz (6.3MB)
    100% |################################| 6.3MB 316kB/s
    Complete output from command python setup.py egg_info:
    ERROR: Sphinx requires at least Python 3.5 to run.

    ----------------------------------------
Command "python setup.py egg_info" failed with error code 1 in /tmp/pip-build-7usNN9/sphinx/
You are using pip version 8.1.1, however version 19.1.1 is available.
You should consider upgrading via the 'pip install --upgrade pip' command.