MAINT: update python=3.8 to python=3.9

Author: mmcky

.github/workflows/cache.yml

@@ -15,7 +15,7 @@ jobs:
           auto-update-conda: true
           auto-activate-base: true
           miniconda-version: 'latest'
-          python-version: 3.8
+          python-version: 3.9
           environment-file: environment.yml
           activate-environment: quantecon
       - name: Build HTML

.github/workflows/execution-linux.yml

@@ -11,7 +11,7 @@ jobs:
       fail-fast: false
       matrix:
         os: ["ubuntu-latest"]
-        python-version: ["3.8"]
+        python-version: ["3.9"]
     steps:
       - name: Checkout
         uses: actions/checkout@v2

.github/workflows/execution-osx.yml

@@ -11,7 +11,7 @@ jobs:
       fail-fast: false
       matrix:
         os: ["macos-latest"]
-        python-version: ["3.8"]
+        python-version: ["3.9"]
     steps:
       - name: Checkout
         uses: actions/checkout@v2

.github/workflows/execution-win.yml

@@ -11,7 +11,7 @@ jobs:
       fail-fast: false
       matrix:
         os: ["windows-latest"]
-        python-version: ["3.8"]
+        python-version: ["3.9"]
     steps:
       - name: Checkout
         uses: actions/checkout@v2

.github/workflows/publish.yml

@@ -16,7 +16,7 @@ jobs:
           auto-update-conda: true
           auto-activate-base: true
           miniconda-version: 'latest'
-          python-version: 3.8
+          python-version: 3.9
           environment-file: environment.yml
           activate-environment: quantecon
       - name: Install latex dependencies