Python: Model MarkupSafe PyPI package

Since expectation tests had so many changes from ConceptsTest, I'm going
to do the changes for that on in a separate commit. The important part
is the changes to taint-tracking, which is highlighted in this commit.

python/ql/test/library-tests/frameworks/markupsafe/ConceptsTest.ql

@@ -1,2 +0,0 @@
-import python
-import experimental.meta.ConceptsTest

python/ql/test/library-tests/frameworks/markupsafe/InlineTaintTest.ql

@@ -1 +1,13 @@
 import experimental.meta.InlineTaintTest
+import semmle.python.Concepts
+
+class HtmlSpecialization extends TestTaintTrackingConfiguration {
+  // TODO: For now, since there is not an `isSanitizingStep` member-predicate part of a
+  // `TaintTracking::Configuration`, we use treat the output is a taint-sanitizer. This
+  // is slightly imprecise, which you can see in the `m_unsafe + SAFE` test-case in
+  // python/ql/test/library-tests/frameworks/markupsafe/taint_test.py
+  //
+  // However, it is better than `getAnInput()`. Due to use-use flow, that would remove
+  // the taint-flow to `SINK()` in `some_escape(tainted); SINK(tainted)`.
+  override predicate isSanitizer(DataFlow::Node node) { node = any(HtmlEscaping esc).getOutput() }
+}

python/ql/test/library-tests/frameworks/markupsafe/taint_test.py

@@ -32,11 +32,11 @@ def test():
 
     ensure_tainted(
         ts, # $ tainted
-        m_unsafe, # $ MISSING: tainted
+        m_unsafe, # $ tainted
         m_unsafe + SAFE, # $ MISSING: tainted
         SAFE + m_unsafe, # $ MISSING: tainted
         m_unsafe.format(SAFE), # $ MISSING: tainted
-        m_unsafe + ts, # $ tainted
+        m_unsafe + ts, # $ MISSING: tainted
 
         m_safe.format(m_unsafe), # $ MISSING: tainted
 
@@ -51,13 +51,13 @@ def test():
         Markup.escape(ts),
 
         m_safe,
-        m_safe + ts, # $ SPURIOUS: tainted
-        ts + m_safe, # $ SPURIOUS: tainted
-        m_safe.format(ts), # $ SPURIOUS: tainted
+        m_safe + ts,
+        ts + m_safe,
+        m_safe.format(ts),
 
-        escape(ts) + ts, # $ SPURIOUS: tainted
-        escape_silent(ts) + ts, # $ SPURIOUS: tainted
-        Markup.escape(ts) + ts, # $ SPURIOUS: tainted
+        escape(ts) + ts,
+        escape_silent(ts) + ts,
+        Markup.escape(ts) + ts,
     )
 
     # flask re-exports these, as:
@@ -66,7 +66,7 @@ def test():
     import flask
 
     ensure_tainted(
-        flask.Markup(ts), # $ MISSING: tainted
+        flask.Markup(ts), # $ tainted
     )
 
     ensure_not_tainted(