doc: Import openslide._version from path added to sys.path

Import _version directly out of the package directory, rather than
changing the path we add to sys.path.  That way we can avoid loading
openslide/__init__.py, which will fail if OpenSlide is not installed
(as when building a downstream package).

Author: bgilbert

doc/conf.py

@@ -52,11 +52,11 @@
 # |version| and |release|, also used in various other places throughout the
 # built documents.
 #
-import openslide._version
+import _version
 # The short X.Y version.
-version = openslide._version.__version__
+version = _version.__version__
 # The full version, including alpha/beta/rc tags.
-release = openslide._version.__version__
+release = _version.__version__
 
 # The language for content autogenerated by Sphinx. Refer to documentation
 # for a list of supported languages.