Fix closure cell rewriting on 3.8 (#539)

* Fix closure cell rewriting on 3.8

* Enable py38

* Don't use CodeType.replace() -- it was added in 3.8.0b1, but CI still uses 3.8.0a4

azure-pipelines.yml

@@ -30,10 +30,9 @@ jobs:
         py37:
           python.version: '3.7'
           tox.env: py37
-        # Our cell rewriting is currently broken on 3.8.
-        # py38:
-        #   python.version: '3.8'
-        #   tox.env: py38
+        py38:
+          python.version: '3.8'
+          tox.env: py38
 
         pypy2:
           python.version: 'pypy2'

src/attr/_compat.py

@@ -164,6 +164,8 @@ def make_set_closure_cell():
         # Convert this code object to a code object that sets the
         # function's first _freevar_ (not cellvar) to the argument.
         args = [co.co_argcount]
+        if sys.version_info >= (3, 8):
+            args.append(co.co_posonlyargcount)
         if not PY2:
             args.append(co.co_kwonlyargcount)
         args.extend(