Allow docs to be built with Sphinx 1.7.1+ (#8500)

* Allow docs to be built with Sphinx 1.7.1+

* Allow docs to be built with Sphinx 2.0+

lib/spack/docs/conf.py

@@ -42,7 +42,13 @@
 import shutil
 import subprocess
 from glob import glob
-from sphinx.apidoc import main as sphinx_apidoc
+
+# Since Sphinx 1.7, sphinx.apidoc has been moved to sphinx.ext.apidoc
+# sphinx.apidoc is deprecated and will be removed in Sphinx 2.0
+try:
+    from sphinx.ext.apidoc import main as sphinx_apidoc
+except ImportError:
+    from sphinx.apidoc import main as sphinx_apidoc
 
 # -- Spack customizations -----------------------------------------------------