gh-35404: Drop support for Python 3.8
    
<!-- Please provide a concise, informative and self-explanatory title.
-->
<!-- Don't put issue numbers in the title. Put it in the Description
below. -->
<!-- For example, instead of "Fixes #12345", use "Add a new method to
multiply two integers" -->

### 📚 Description

In preparation for important package upgrades, we remove support for
Python v3.8. (Almost) all occurrences of "Python 3.8" are removed from
the codebase. There are still some comments left that essentially say
that in newer versions things can be simplified, but I didn't touch
those and leave them for follow-up PRs.

This will help with:
-  PRs that require Python 3.9 or above, such as
#34973 and
#35188.
- Upgrades of Python packages that have dropped support for Python 3.8
because they follow NEP 29 and have already released a new major version
that drops support according to the NEP 29 schedule.
  - NetworkX 3.2 (expected 2023-??; version 3.1 was released 2023-04,
see #35671)
  - #34816
  - #35703
  - See also #32074.

Test run: https://github.com/tobiasdiez/sage/actions/runs/4586981626

<!-- Describe your changes here in detail. -->
<!-- Why is this change required? What problem does it solve? -->
<!-- If this PR resolves an open issue, please link to it here. For
example "Fixes #12345". -->
<!-- If your change requires a documentation PR, please link it
appropriately. -->

### 📝 Checklist

<!-- Put an `x` in all the boxes that apply. It should be `[x]` not `[x
]`. -->

- [x] The title is concise, informative, and self-explanatory.
- [x] The description explains in detail what this PR is about.
- [ ] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [x] I have updated the documentation accordingly.

### ⌛ Dependencies

<!-- List all open PRs that this PR logically depends on
- #12345: short description why this is a dependency
- #34567: ...
-->

<!-- If you're unsure about any of these, don't hesitate to ask. We're
here to help! -->
    
URL: #35404
Reported by: Tobias Diez
Reviewer(s): Dima Pasechnik, Matthias Köppe

Author: Release Manager

.github/workflows/dist.yml

@@ -100,7 +100,7 @@ jobs:
       #
       CIBW_ARCHS: ${{ matrix.arch }}
       # https://cibuildwheel.readthedocs.io/en/stable/options/#requires-python
-      CIBW_PROJECT_REQUIRES_PYTHON: ">=3.8"
+      CIBW_PROJECT_REQUIRES_PYTHON: ">=3.9"
       # Environment during wheel build
       CIBW_ENVIRONMENT: "PATH=$(pwd)/local/bin:$PATH CPATH=$(pwd)/local/include:$CPATH LIBRARY_PATH=$(pwd)/local/lib:$LIBRARY_PATH PKG_CONFIG_PATH=$(pwd)/local/share/pkgconfig:$PKG_CONFIG_PATH ACLOCAL_PATH=/usr/share/aclocal"
       # Use 'build', not 'pip wheel'

.github/workflows/docker.yml

@@ -18,7 +18,6 @@ on:
         default: >-
           ["ubuntu-trusty-toolchain-gcc_9",
            "ubuntu-xenial-toolchain-gcc_9",
-           "ubuntu-bionic-gcc_8-python3.8",
            "ubuntu-focal",
            "ubuntu-jammy",
            "ubuntu-kinetic",
@@ -28,16 +27,13 @@ on:
            "debian-bullseye",
            "debian-bookworm",
            "debian-sid",
-           "linuxmint-19-gcc_8-python3.8",
-           "linuxmint-19.3-gcc_8-python3.8",
            "linuxmint-20.1",
            "linuxmint-20.2",
            "linuxmint-20.3",
            "linuxmint-21",
            "linuxmint-21.1",
            "linuxmint-21.2",
-           "fedora-29-python3.8",
-           "fedora-30-python3.8",
+           "fedora-30",
            "fedora-31",
            "fedora-32",
            "fedora-33",

.github/workflows/lint.yml

@@ -17,7 +17,7 @@ jobs:
     - name: Set up Python
       uses: actions/setup-python@v4
       with:
-        python-version: 3.8
+        python-version: 3.9
     - name: Install pycodestyle
       run: pip install tox pycodestyle
     - name: Lint using pycodestyle
@@ -31,7 +31,7 @@ jobs:
     - name: Set up Python
       uses: actions/setup-python@v4
       with:
-        python-version: 3.8
+        python-version: 3.9
     - name: Install relint
       run: pip install tox relint
     - name: Lint using relint
@@ -45,7 +45,7 @@ jobs:
     - name: Set up Python
       uses: actions/setup-python@v4
       with:
-        python-version: 3.8
+        python-version: 3.9
     - name: Install tox
       run: pip install tox
     - name: Lint using tox -e rst

README.md

@@ -201,7 +201,7 @@ in the Installation Guide.
       more details.
 
     - Python 3.4 or later, or Python 2.7, a full installation including
-      `urllib`; but ideally version 3.8.x, 3.9.x, or 3.10.x, which
+      `urllib`; but ideally version 3.9.x, 3.10.x, or 3.11.x, which
       will avoid having to build Sage's own copy of Python 3.
       See [build/pkgs/python3/SPKG.rst](build/pkgs/python3/SPKG.rst)
       for more details.