pipeline: no longer test examples on python3.6; relock examples env with pipenv v2022.1.8 on python v3.7.12

.github/workflows/python.yml

@@ -125,7 +125,6 @@ jobs:
     strategy:
       matrix:
         python-version:
-        - '3.6'
         - '3.7'
         - '3.8'
         - '3.9'

CHANGELOG.md

@@ -10,6 +10,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Removed
 - compatibility with `python3.5`
+- examples: drop compatibility with `python3.6`
 
 ## [2.1.1] - 2021-06-11
 ### Fixed

examples/Pipfile

@@ -12,16 +12,5 @@ notebook = "*"
 # nbconvert v6.0.2 made `;` suffix on last line ineffective
 nbconvert = "<6.0.2"
 
-# workaround pipenv expectedly adding "python_version >= '3.7'" marker:
-# >   File "/home/runner/.local/share/virtualenvs/examples-bH_l4rXc/lib/python3.5/site-packages/nbconvert/exporters/base.py", line 10, in <module>
-# >     from traitlets.log import get_logger
-# > ImportError: No module named 'traitlets'
-traitlets = "*"
-# workaround pipenv adding "python_full_version >= '3.6.1'" marker:
-# >   File "/home/runner/.local/share/virtualenvs/examples-bH_l4rXc/lib/python3.5/site-packages/IPython/terminal/interactiveshell.py", line 18, in <module>
-# >     from prompt_toolkit.enums import DEFAULT_BUFFER, EditingMode
-# > ImportError: No module named 'prompt_toolkit'
-prompt-toolkit = "*"
-
 [requires]
 python_version = "3"