Skip sanic newest on python 3.8

Because it fails with:

/lib/python3.8/site-packages/sanic/helpers.py:21: in <module>
    STATUS_CODES: dict[int, bytes] = {
E   TypeError: 'type' object is not subscriptabl

Author: xrmx

.ci/.matrix_exclude.yml

@@ -217,6 +217,8 @@ exclude:
     FRAMEWORK: sanic-20.12
   - VERSION: python-3.6
     FRAMEWORK: sanic-newest
+  - VERSION: python-3.8
+    FRAMEWORK: sanic-newest
   - VERSION: pypy-3
     # aioredis
     FRAMEWORK: aioredis-newest