Move to Python 3.7 as a minimum version

The term-image library, needed for image display, is Python 3.7+
so I have made what I think are the minimum set of changes to
move to Python 3.7 - testing with 3.7+ only, as a start.

Unchanged- vermin target is still 3.6 because changing it to 3.7
fails for an unknown reason. Seems harmless to leave it at 3.6
for the time being.

.github/workflows/test.yml

@@ -4,12 +4,10 @@ on: [push, pull_request]
 
 jobs:
   test:
-    # Older Ubuntu required for testing on Python 3.6 which is not available in
-    # later versions. Remove once support for 3.6 is dropped.
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-22.04
     strategy:
       matrix:
-        python-version: ["3.6", "3.7", "3.8", "3.9", "3.10", "3.11"]
+        python-version: ["3.7", "3.8", "3.9", "3.10", "3.11"]
 
     steps:
       - uses: actions/checkout@v3

requirements.txt

@@ -2,5 +2,5 @@ requests>=2.13,<3.0
 beautifulsoup4>=4.5.0,<5.0
 wcwidth>=0.1.7
 urwid>=2.0.0,<3.0
-pillow>=8.4.0
+pillow>=9.5.0
 term-image==0.6.0

setup.py

@@ -32,13 +32,13 @@ setup(
         'Programming Language :: Python :: 3',
     ],
     packages=['toot', 'toot.tui', 'toot.utils'],
-    python_requires=">=3.6",
+    python_requires=">=3.7",
     install_requires=[
         "requests>=2.13,<3.0",
         "beautifulsoup4>=4.5.0,<5.0",
         "wcwidth>=0.1.7",
         "urwid>=2.0.0,<3.0",
-        "pillow>=8.4.0",
+        "pillow>=9.5.0",
         "term-image==0.6.0",
     ],
     entry_points={