Raise minimum python version to 3.7

- Ubuntu LTS (version 22) min python is now 3.7
- We get better formatters
- Latest version of click dropped support for Python 3.6

After this change, we can start a round of code cleaning in prep for a
1.0.

This change revert the Ubuntu target to ubuntu-latest and drops 3.6 from
the automated tests.

.github/workflows/core-tests.yml

@@ -4,13 +4,13 @@ on: [push, pull_request]
 
 jobs:
   build:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     strategy:
       max-parallel: 3
       matrix:
-        # This is currently overkill since we are targeting 3.5
-        # but affords us visibility onto syntax changes in newer Pythons
-        python-version: [3.6, 3.7, 3.8, 3.9]
+        # This range ensures coding standard does not conflict with 
+        #  changes in newer Pythons
+        python-version: [3.7, 3.8, 3.9, 3.10]
 
     steps:
     - uses: actions/checkout@v3

piku.py

@@ -4,9 +4,9 @@
 
 try:
     from sys import version_info
-    assert version_info >= (3, 5)
+    assert version_info >= (3, 7)
 except AssertionError:
-    exit("Piku requires Python 3.5 or above")
+    exit("Piku requires Python 3.7 or above")
 
 from importlib import import_module
 from collections import defaultdict, deque