Drop Python 3.8 support

This addresses a failure in the `py38-dev` tests
caused by markupsafe recently dropping Python 3.8 support.

Changed in this commit:

* Drop Python 3.8 support
* Update CI and tox testing
* Update docs references to Python 3.8
* Remove a Windows / Python 3.8 admonition in the docs
* Address type annotations
* Incorporate style fixes

Still to do:

* `src/flask/sessions.py` contains these comments:

  > TODO generic when Python > 3.8

  This TODO should now be unblocked.

pyproject.toml

@@ -19,7 +19,7 @@ classifiers = [
     "Topic :: Software Development :: Libraries :: Application Frameworks",
     "Typing :: Typed",
 ]
-requires-python = ">=3.8"
+requires-python = ">=3.9"
 dependencies = [
     "Werkzeug>=3.0.0",
     "Jinja2>=3.1.2",
@@ -78,7 +78,7 @@ source = ["flask", "tests"]
 source = ["src", "*/site-packages"]
 
 [tool.mypy]
-python_version = "3.8"
+python_version = "3.9"
 files = ["src/flask", "tests/typing"]
 show_error_codes = true
 pretty = true
@@ -94,7 +94,7 @@ module = [
 ignore_missing_imports = true
 
 [tool.pyright]
-pythonVersion = "3.8"
+pythonVersion = "3.9"
 include = ["src/flask", "tests"]
 typeCheckingMode = "basic"