Update Linux CI scripts, more Python versions.

Update scripts to use actions/setup-python to install different Python
versions.  Add run-faber.sh and get-py-env.py scripts.  Add
test-ubuntu-py-ver.yml CI script to test with different Python versions.

.github/get-py-env.py

@@ -0,0 +1,65 @@
+#!/usr/bin/env python3
+#
+# Determine info about the Python install and write shell code to stdout, to
+# set env variables.  This will set the variables PY_LDFLAGS, PY_CFLAGS and
+# PY_INC_PATH.
+#
+# The python3-config tool is used as the source of this info. In theory we
+# could use sysconfig as well but the setup-python action from github appears
+# to patch python3-config but not patch the sysconfig info.
+#
+# Usage:
+#   eval $(python3 get-py-env.py)
+
+import os
+import re
+import subprocess
+
+
+def get_output(cmd):
+    rv = subprocess.run(
+        cmd,
+        capture_output=True,  # Capture stdout and stderr
+        text=True,  # Decode output as text (UTF-8)
+        check=True,  # Raise an error if the command fails
+    )
+    return rv.stdout
+
+
+def extract_flags(cmd, prefix):
+    flags = []
+    for part in get_output(cmd).split():
+        part = part.strip()
+        if part.startswith(prefix):
+            flags.append(part)
+    return ' '.join(flags)
+
+
+def find_python_h():
+    """Find the include path that has Python.h contained inside.
+    We could use INCLUDEPY from sysconfig but github patches
+    python3-config but not the sysconfig info (after moving the
+    install).
+    """
+    c_flags = extract_flags(['python3-config', '--cflags'], '-I')
+    for part in c_flags.split():
+        m = re.search(r'-I(\S+)', part)
+        if not m:
+            continue
+        inc_path = m.group(1)
+        if os.path.exists(os.path.join(inc_path, 'Python.h')):
+            return inc_path
+    raise SystemExit('cannot find Python.h')
+
+
+def main():
+    ld_flags = extract_flags(['python3-config', '--ldflags'], '-L')
+    c_flags = extract_flags(['python3-config', '--cflags'], '-I')
+    include_path = find_python_h()
+    print(f'PY_LDFLAGS="{ld_flags}"')
+    print(f'PY_CFLAGS="{c_flags}"')
+    print(f'PY_INC_PATH="{include_path}"')
+
+
+if __name__ == '__main__':
+    main()

.github/run-faber.sh

@@ -0,0 +1,46 @@
+#!/bin/sh
+
+set -eu
+
+echo "cxx version: $CXX $($CXX --version)"
+echo "cxx std: $CXX_STD"
+echo "python3 path: $(which python3)"
+echo "python3 version: $(python3 --version)"
+
+if ! which faber > /dev/null; then
+    echo "Installing faber..."
+    python3 -m pip install --upgrade pip
+    python3 -m pip install -U faber
+fi
+echo "faber version: $(faber -v)"
+
+# find and set PY_LDFLAGS and PY_INC_PATH
+eval $(python3 .github/get-py-env.py)
+
+echo "PY_INC_PATH=$PY_INC_PATH"
+echo "PY_LDFLAGS=$PY_LDFLAGS"
+
+case $(python3-config --abiflags) in
+    *t*)
+        # When running with free-threaded, we always want to disable the GIL
+        # even for extensions without the mod_gil_not_used() flag
+        export PYTHON_GIL=0
+        ;;
+esac
+
+# this could be set by LD_LIBRARY_PATH but faber overrides it
+prefix=$(python3-config --prefix)
+echo "${prefix}/lib" > /etc/ld.so.conf.d/boost-ci.conf && ldconfig
+
+sed -e "s/\$PYTHON/python3/g" .ci/faber > $HOME/.faber
+
+faber \
+  --with-boost-include=${BOOST_PY_DEPS} \
+  --builddir=build \
+  cxx.name="${CXX}" \
+  cxxflags="-std=${CXX_STD}" \
+  cppflags="-std=${CXX_STD}" \
+  include="${PY_INC_PATH}" \
+  ldflags="${PY_LDFLAGS}" \
+  -j`nproc` \
+  "$@"

.github/workflows/test-ubuntu.yml

@@ -1,6 +1,10 @@
+# Test on Ubuntu with various compiler and language standard versions.
 name: Test Ubuntu
 
-on:  [push, pull_request]
+on:
+  push:
+  pull_request:
+  workflow_dispatch:
 
 jobs:
   build:
@@ -9,7 +13,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python: [python, python3]
+        python-version: ['3.14']
         cxx: [g++, clang++]
         std: [c++11, c++14, c++17]
         include:
@@ -27,31 +31,17 @@ jobs:
 
     steps:
     - uses: actions/checkout@v5
-
+    - name: setup python
+      uses: actions/setup-python@v5
+      with:
+        python-version: ${{ matrix.python-version }}
     - name: setup prerequisites
       run: |
-        # Warning: this is not necessarily the same Python version as the one configured above !
-        python3 -m pip install -U faber --break-system-packages
+        echo "CXX=${{ matrix.cxx }}" >> "$GITHUB_ENV"
+        echo "CXX_STD=${{ matrix.std }}" >> "$GITHUB_ENV"
     - name: build
       run: |
-        ${{ matrix.python }} --version
-        ${{ matrix.cxx }} --version
-        faber -v
-        sed -e "s/\$PYTHON/${{ matrix.python }}/g" .ci/faber > ~/.faber
-        faber \
-          --with-boost-include=${BOOST_PY_DEPS} \
-          --builddir=build \
-          cxx.name=${{ matrix.cxx }} \
-          cxxflags=-std=${{ matrix.std }} \
-          cppflags=-std=${{ matrix.std }} \
-          -j`nproc`
+        .github/run-faber.sh
     - name: test
       run: |
-        faber \
-          --with-boost-include=${BOOST_PY_DEPS} \
-          --builddir=build \
-          cxx.name=${{ matrix.cxx }} \
-          cxxflags=-std=${{ matrix.std }} \
-          cppflags=-std=${{ matrix.std }} \
-          -j`nproc` \
-          test.report
+        .github/run-faber.sh test.report