CI: enable testing on 3.9

Author: stanislaw

.github/workflows/ci-linux.yml

@@ -8,7 +8,7 @@ jobs:
 
     strategy:
       matrix:
-        python-version: [3.5, 3.6, 3.7, 3.8]
+        python-version: [3.5, 3.6, 3.7, 3.8, 3.9]
 
     steps:
     - uses: actions/checkout@v2

.github/workflows/ci-mac.yml

@@ -8,7 +8,7 @@ jobs:
 
     strategy:
       matrix:
-        python-version: [3.5, 3.6, 3.7, 3.8]
+        python-version: [3.5, 3.6, 3.7, 3.8, 3.9]
 
     steps:
     - uses: actions/checkout@v2

.github/workflows/ci-windows.yml

@@ -8,7 +8,7 @@ jobs:
 
     strategy:
       matrix:
-        python-version: [3.5, 3.6, 3.7, 3.8]
+        python-version: [3.5, 3.6, 3.7, 3.8, 3.9]
 
     steps:
     - uses: actions/checkout@v2

.python-version

@@ -1,2 +1,2 @@
-3.5.0
+3.6.2