Fix pynamodb tests for Python < 3.6

PynamoDB 4.4.0 introduced syntax that's only valid on Python >= 3.6
while still claiming to support Python < 3.6.
See pynamodb/PynamoDB#1130

Author: michael-k

tox.ini

@@ -104,7 +104,9 @@ deps =
     ext-django-4: Django >=4.0,<5.0
     ext-django: django-fake-model
 
-    ext-pynamodb: pynamodb >= 3.3.1
+    py{27,34,35}-ext-pynamodb: pynamodb >=3.3.1,<4.4
+    py{27,34,35}-ext-pynamodb: botocore <1.28
+    py{36,37,38,39}-ext-pynamodb: pynamodb >=3.3.1
 
     ext-psycopg2: psycopg2
     ext-psycopg2: testing.postgresql