treewide: make v3.8 the minimum supported Python

Upstream Zephyr has moved to python v3.8 as a minimum version, so it's
OK for west to move too. Make that happen.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>

.github/workflows/test.yml

@@ -9,7 +9,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu-latest, macos-latest, windows-latest]
-        python-version: [3.7, 3.8, 3.9]
+        python-version: [3.8, 3.9, '3.10']
     steps:
       - uses: actions/checkout@v1
       - name: Set up Python

setup.py

@@ -47,6 +47,6 @@ setuptools.setup(
         'setuptools',
         'packaging',
     ],
-    python_requires='>=3.6',
+    python_requires='>=3.8',
     entry_points={'console_scripts': ('west = west.app.main:main',)},
 )