[7.x] Add Python 3.9 to CI

Co-authored-by: Seth Michael Larson <seth.larson@elastic.co>

.ci/test-matrix.yml

@@ -12,6 +12,7 @@ PYTHON_VERSION:
   - 3.6
   - 3.7
   - 3.8
+  - 3.9
 
 PYTHON_CONNECTION_CLASS:
   - Urllib3HttpConnection

.github/workflows/ci.yml

@@ -60,7 +60,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: [2.7, 3.5, 3.6, 3.7, 3.8]
+        python-version: [2.7, 3.5, 3.6, 3.7, 3.8, 3.9]
 
     steps:
       - name: Checkout Repository

noxfile.py

@@ -27,7 +27,7 @@ SOURCE_FILES = (
 )
 
 
-@nox.session(python=["2.7", "3.4", "3.5", "3.6", "3.7", "3.8"])
+@nox.session(python=["2.7", "3.4", "3.5", "3.6", "3.7", "3.8", "3.9"])
 def test(session):
     session.install(".")
     session.install("-r", "dev-requirements.txt")