Use relative import.

[jmbr]

This patch fixes the error shown below:

---------------------------------------------------------------------------
ModuleNotFoundError Traceback (most recent call last)
<ipython-input-1-ef99ef88c3de> in <module>
----> 1 import phaseplot
/workspace/tools/languages/python/python-phaseplot/phaseplot/__init__.py in <module>
----> 1 from plotfuns import phase_portrait
ModuleNotFoundError: No module named 'plotfuns'