Make alabaster and sphinx_rtd_theme dependencies optional

There is no need to pull these dependencies if i.e. the documentation is
using the classic theme.

There will be no change in behavior for projects using alabaster or
sphinx_rtd_theme.

sphinx/theming.py

@@ -26,9 +26,6 @@
 from sphinx import package_dir
 from sphinx.errors import ThemeError
 
-import alabaster
-import sphinx_rtd_theme
-
 NODEFAULT = object()
 THEMECONF = 'theme.conf'
 
@@ -73,10 +70,12 @@ def init_themes(cls, confdir, theme_path, warn=None):
     def load_extra_theme(cls, name):
         if name in ('alabaster', 'sphinx_rtd_theme'):
             if name == 'alabaster':
+                import alabaster
                 themedir = alabaster.get_path()
                 # alabaster theme also requires 'alabaster' extension, it will be loaded
                 # at sphinx.application module.
             elif name == 'sphinx_rtd_theme':
+                import sphinx_rtd_theme
                 themedir = sphinx_rtd_theme.get_html_theme_path()
             else:
                 raise NotImplementedError('Programming Error')