installing ppextensions in ubuntu 18.04 (python3.6)

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when I try installing ppextensions I get the error below when the system executes pip.get_installed_distributions(). Any clue?

...

Collecting apache-airflow==1.8.2
  Downloading apache-airflow-1.8.2.tar.gz (2.3 MB)
     |████████████████████████████████| 2.3 MB 2.6 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-jnqwu9o_/apache-airflow/setup.py'"'"'; __file__='"'"'/tmp/pip-install-jnqwu9o_/apache-airflow/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-jnqwu9o_/apache-airflow/pip-egg-info
         cwd: /tmp/pip-install-jnqwu9o_/apache-airflow/
    Complete output (9 lines):
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "/tmp/pip-install-jnqwu9o_/apache-airflow/setup.py", line 302, in <module>
        do_setup()
      File "/tmp/pip-install-jnqwu9o_/apache-airflow/setup.py", line 198, in do_setup
        check_previous()
      File "/tmp/pip-install-jnqwu9o_/apache-airflow/setup.py", line 105, in check_previous
        in pip.get_installed_distributions()])
    AttributeError: module 'pip' has no attribute 'get_installed_distributions'
    ----------------------------------------
ERROR: Command errored out with exit status 1: python setup.py egg_info Check the logs for full command output.