fix Python install issues

[vladak]

currently all Linux builds fail due to (from pep8 install):

Collecting distlib<1,>=0.3.0
  Downloading distlib-0.3.0.zip (571 kB)
     |████████████████████████████████| 571 kB 43.4 MB/s 
    ERROR: Command errored out with exit status 1:
     command: /usr/bin/python3 -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'/tmp/pip-install-n_2nuwn0/distlib/setup.py'"'"'; __file__='"'"'/tmp/pip-install-n_2nuwn0/distlib/setup.py'"'"';f=getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' egg_info --egg-base /tmp/pip-install-n_2nuwn0/distlib/pip-egg-info
         cwd: /tmp/pip-install-n_2nuwn0/distlib/
    Complete output (3 lines):
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
    ImportError: No module named 'setuptools'

so evidently we need to install setuptools beforehand.

Further, pip install complains:

WARNING: The directory '/home/travis/.cache/pip' or its parent directory is not owned or is not writable by the current user. The cache has been disabled. Check the permissions and owner of that directory. If executing pip with sudo, you may want sudo's -H flag.

so we need to let sudo to set HOME env var.