Dependency Specifiers: Don't require whitespace after URL

otherwise would `numpy @ https://example.org/numpy` is not a valid specifier (missing whitespace after the URL). This already what pip and packaging do and also what the parsley grammar says

Author: konstin

source/specifications/dependency-specifiers.rst

@@ -113,7 +113,7 @@ Giving us a rule for name based requirements::
 
 And a rule for direct reference specifications::
 
-    url_req       = name wsp* extras? wsp* urlspec wsp+ quoted_marker?
+    url_req       = name wsp* extras? wsp* urlspec (wsp+ quoted_marker?)?
 
 Leading to the unified rule that can specify a dependency.::