Move doc theme to alabaster

Reason is that the RTD theme looks weird for multi-paragraph parameter API
docs.

CHANGELOG.rst

@@ -44,6 +44,7 @@ Changes:
   Check out the `examples <https://attrs.readthedocs.io/en/stable/examples.html#validators>`_ to see it in action!
   `#143 <https://github.com/hynek/attrs/issues/143>`_
 
+
 ----
 
 

docs/conf.py

@@ -1,26 +1,9 @@
 # -*- coding: utf-8 -*-
-#
-# attrs documentation build configuration file, created by
-# sphinx-quickstart on Sun May 11 16:17:15 2014.
-#
-# This file is execfile()d with the current directory set to its
-# containing dir.
-#
-# Note that not all possible configuration values are present in this
-# autogenerated file.
-#
-# All configuration values have a default; values that are commented out
-# serve to show the default.
 
 import codecs
 import os
 import re
 
-try:
-    import sphinx_rtd_theme
-except ImportError:
-    sphinx_rtd_theme = None
-
 
 def read(*parts):
     """
@@ -134,11 +117,7 @@ def find_version(*file_paths):
 # The theme to use for HTML and HTML Help pages.  See the documentation for
 # a list of builtin themes.
 
-if sphinx_rtd_theme:
-    html_theme = "sphinx_rtd_theme"
-    html_theme_path = [sphinx_rtd_theme.get_html_theme_path()]
-else:
-    html_theme = "default"
+html_theme = "alabaster"
 
 # Theme options are theme-specific and customize the look and feel of a theme
 # further.  For a list of options available for each theme, see the