Remove redundant import from sphinx conf

[gyermolenko]

This PR is intended to fix #284 . RTD builds are failing for quite some time with

Exception occurred:
  File "conf.py", line 158, in <module>
    html_theme_path = [alabaster.get_path()]
NameError: name 'alabaster' is not defined

According to alabaster docs this config option is redundant since Sphinx 1.3.

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  Hits              ?      200           
  Misses            ?        4           
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[asvetlov]

thanks!