Prevent compile error when the result of repr() is assigned to a "unicode" LHS...

Prevent compile error when the result of repr() is assigned to a "unicode" LHS with language_level=3.
Closes https://github.com/cython/cython/issues/3736

CHANGES.rst

@@ -2,6 +2,16 @@
 Cython Changelog
 ================
 
+0.29.22 (2020-??-??)
+====================
+
+Bugs fixed
+----------
+
+* ``repr()`` was assumed to return ``str`` instead of ``unicode`` with ``language_level=3``.
+  (Github issue #3736)
+
+
 0.29.21 (2020-07-09)
 ====================
 

Cython/Compiler/Builtin.py

@@ -203,7 +203,7 @@ builtin_function_table = [
     #('raw_input', "",     "",      ""),
     #('reduce',    "",     "",      ""),
     BuiltinFunction('reload',     "O",    "O",     "PyImport_ReloadModule"),
-    BuiltinFunction('repr',       "O",    "O",     "PyObject_Repr", builtin_return_type='str'),
+    BuiltinFunction('repr',       "O",    "O",     "PyObject_Repr"),  # , builtin_return_type='str'),  # add in Cython 3.1
     #('round',     "",     "",      ""),
     BuiltinFunction('setattr',    "OOO",  "r",     "PyObject_SetAttr"),
     #('sum',       "",     "",      ""),

tests/run/cython3.pyx

@@ -630,3 +630,11 @@ async def async_def_annotations(x: 'int') -> 'float':
     int
     """
     return float(x)
+
+
+def repr_returns_str(x) -> str:
+    """
+    >>> repr_returns_str(123)
+    '123'
+    """
+    return repr(x)