sagemath launch problems on Ubuntu

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I think I have a SageMath environment variable setting problem causing to SageMath to throw Python exceptions at startup.

Here are my software versions:

vasily@vasily-Latitude-E7240:~$ sage --version
SageMath version 8.1, Release Date: 2017-12-07
vasily@vasily-Latitude-E7240:~$ python --version
Python 2.7.15rc1
vasily@vasily-Latitude-E7240:~$ uname -a
Linux vasily-Latitude-E7240 4.15.0-47-generic #50-Ubuntu SMP Wed Mar 13 10:44:52 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux
vasily@vasily-Latitude-E7240:~$ uname -v
#50-Ubuntu SMP Wed Mar 13 10:44:52 UTC 2019

Here is the Python backtrace:

Traceback (most recent call last):
  File "/usr/share/sagemath/bin/sage-ipython", line 7, in <module>
    from sage.repl.interpreter import SageTerminalApp
  File "/usr/lib/python2.7/dist-packages/sage/repl/interpreter.py", line 107, in <module>
    from sage.repl.preparse import preparse
  File "/usr/lib/python2.7/dist-packages/sage/repl/preparse.py", line 225, in <module>
    import six
ImportError: No module named six
Traceback (most recent call last):
  File "/usr/share/sagemath/bin/sage-ipython", line 7, in <module>
    from sage.repl.interpreter import SageTerminalApp
  File "/usr/lib/python2.7/dist-packages/sage/repl/interpreter.py", line 107, in <module>
    from sage.repl.preparse import preparse
  File "/usr/lib/python2.7/dist-packages/sage/repl/preparse.py", line 225, in <module>
    import six
ImportError: No module named six

Somehow preparse.py is not happy. Any ideas?