make exception __repr__ compatible with python3.6

Author: mkurnikov

tests/snippets/exceptions.py

@@ -6,7 +6,7 @@
 
 exc = KeyError('message')
 assert str(exc) == "'message'"
-assert repr(exc) == "KeyError('message')"
+assert repr(exc) == "KeyError('message',)"
 
 exc = KeyError('message', 'another message')
 assert str(exc) == "('message', 'another message')"
@@ -22,4 +22,4 @@ def __str__(self):
 
 exc = KeyError(A())
 assert str(exc) == 'repr'
-assert repr(exc) == 'KeyError(repr)'
+assert repr(exc) == 'KeyError(repr,)'

vm/src/exceptions.rs

@@ -185,6 +185,7 @@ fn exception_repr(vm: &VirtualMachine, args: PyFuncArgs) -> PyResult {
     let exc_name = exc.class().name.clone();
     let joined_str = match args_repr.len() {
         0 => format!("{}()", exc_name),
+        1 => format!("{}({},)", exc_name, args_repr[0]),
         _ => format!("{}({})", exc_name, args_repr.join(", ")),
     };
     Ok(vm.new_str(joined_str))