Python: Add up-/downgrade scripts

In the upgrade direction, we simply do nothing.

In the downgrade direction, we remove the two new relations, and
also any `Stmt` nodes corresponding to `TypeAlias` nodes.

python/downgrades/728c6d65e61d808ae276013ebc15abc3a97aaef1/py_stmts.ql

@@ -0,0 +1,18 @@
+// We must wrap the DB types, as these cannot appear in argument lists
+class Stmt_ extends @py_stmt {
+  string toString() { result = "Stmt" }
+}
+
+class StmtList_ extends @py_stmt_list {
+  string toString() { result = "StmtList" }
+}
+
+query predicate py_stmts_without_typealias(Stmt_ id, int kind, StmtList_ parent, int idx) {
+  py_stmts(id, kind, parent, idx) and
+  // From the dbscheme:
+  //
+  // case @py_stmt.kind of
+  // ...
+  // |   27 = @py_TypeAlias;
+  kind != 27
+}

python/downgrades/728c6d65e61d808ae276013ebc15abc3a97aaef1/upgrade.properties

@@ -0,0 +1,5 @@
+description: Remove support for type parameters and type alias statements
+compatibility: backwards
+py_type_parameters.rel: delete
+py_type_parameter_lists.rel: delete
+py_stmts.rel: run py_stmts.qlo py_stmts_without_typealias

python/ql/lib/upgrades/0565f7466437d52e1dc64a3b930926ab2f60cd64/upgrade.properties

@@ -0,0 +1,2 @@
+description: Add support for type parameters and type alias statements
+compatibility: backwards