Python: Add up-/downgrade scripts for template literals

We do the usual thing. Downgrade scripts remove the relevant relations;
upgrade scripts do nothing.

python/downgrades/8d257a4a9bc78e39856d6cd33499389fc5148d4f/py_exprs.ql

@@ -0,0 +1,21 @@
+// We must wrap the DB types, as these cannot appear in argument lists
+class Expr_ extends @py_expr {
+  string toString() { result = "Expr" }
+}
+
+class ExprParent_ extends @py_expr_parent {
+  string toString() { result = "ExprList" }
+}
+
+query predicate py_exprs_without_template_strings(Expr_ id, int kind, ExprParent_ parent, int idx) {
+  py_exprs(id, kind, parent, idx) and
+  // From the dbscheme:
+  //
+  // case @py_expr.kind of
+  // ...
+  // |   39 = @py_SpecialOperation
+  // |   40 = @py_TemplateString
+  // |   41 = @py_JoinedTemplateString
+  // |   42 = @py_TemplateStringPart;
+  not kind in [40, 41, 42]
+}

python/downgrades/8d257a4a9bc78e39856d6cd33499389fc5148d4f/upgrade.properties

@@ -0,0 +1,4 @@
+description: Remove support for template string literals
+compatibility: backwards
+py_TemplateString_lists.rel: delete
+py_exprs.rel: run py_exprs.qlo py_exprs_without_template_strings

python/ql/lib/upgrades/acf8d3b08ae3cfac8833d16efbfa5a10fef86819/upgrade.properties

@@ -0,0 +1,2 @@
+description: Add support for template string literals
+compatibility: backwards