[mypyc] Support ellipsis (...) expressions in class bodies

This can be used to declare concise custom exceptions, e.g.

    class UnknownReleaseError(ValueError): ...

which otherwise probably would be written

    class UnknownReleaseError(ValueError):
        pass

and is supported by CPython.

Closes mypyc/mypyc#1069

Author: Newbytee

mypyc/irbuild/classdef.py

@@ -12,6 +12,7 @@
     CallExpr,
     ClassDef,
     Decorator,
+    EllipsisExpr,
     ExpressionStmt,
     FuncDef,
     Lvalue,
@@ -145,7 +146,11 @@ def transform_class_def(builder: IRBuilder, cdef: ClassDef) -> None:
                 continue
             with builder.catch_errors(stmt.line):
                 cls_builder.add_method(get_func_def(stmt))
-        elif isinstance(stmt, PassStmt):
+        elif (
+            isinstance(stmt, PassStmt)
+            or isinstance(stmt, ExpressionStmt)
+            and isinstance(stmt.expr, EllipsisExpr)
+        ):
             continue
         elif isinstance(stmt, AssignmentStmt):
             if len(stmt.lvalues) != 1: