Python: Add Unit helper library

Like Java did in https://github.com/github/codeql/pull/4184

python/ql/src/python.qll

@@ -35,6 +35,7 @@ import semmle.python.pointsto.Base
 import semmle.python.pointsto.Context
 import semmle.python.pointsto.CallGraph
 import semmle.python.objects.ObjectAPI
+import semmle.python.Unit
 import site
 // Removing this import perturbs the compilation process enough that the points-to analysis gets
 // compiled -- and cached -- differently depending on whether the data flow library is imported. By

python/ql/src/semmle/python/Unit.qll

@@ -0,0 +1,10 @@
+/** Provides the `Unit` class. */
+
+/** The unit type. */
+private newtype TUnit = TMkUnit()
+
+/** The trivial type with a single element. */
+class Unit extends TUnit {
+  /** Gets a textual representation of this element. */
+  string toString() { result = "unit" }
+}

python/ql/src/semmle/python/dataflow/new/internal/TaintTrackingPublic.qll

@@ -35,15 +35,6 @@ predicate localTaintStep(DataFlow::Node nodeFrom, DataFlow::Node nodeTo) {
   localAdditionalTaintStep(nodeFrom, nodeTo)
 }
 
-// AdditionalTaintStep for global taint flow
-private newtype TUnit = TMkUnit()
-
-/** A singleton class containing a single dummy "unit" value. */
-private class Unit extends TUnit {
-  /** Gets a textual representation of this element. */
-  string toString() { result = "unit" }
-}
-
 /**
  * A unit class for adding additional taint steps.
  *