.github/workflows/tox*.yml: Update platforms for drop of GCC < 6.3

ubuntu-trusty 14.04 is long past standard support, but ​EOL has been extended to Apr 2024. Replace by ubuntu-trusty-toolchain-gcc_11
centos-7 has EOL June 2024, gcc 4.x. Replace by centos-7-devtoolset-gcc_11
linuxmint-17 had ​EOL 2019. gcc 4.x. Drop
linuxmint-18 had ​EOL 2021, gcc 5.4. Drop
slackware-14.2 with gcc 5.5. Drop

.github/workflows/tox-experimental.yml

@@ -38,10 +38,7 @@ jobs:
       fail-fast: false
       max-parallel: 6
       matrix:
-        # This list is different from the one in tox.yml:
-        # Trac #31526 switches gcc 4.x-based distributions to using the gcc_spkg configuration factor
-        # Trac #32281 removes gcc 4.x-based distributions whose binutils are unusable
-        tox_system_factor: [ubuntu-xenial, ubuntu-bionic, ubuntu-focal, ubuntu-hirsute, ubuntu-impish, ubuntu-jammy, debian-stretch, debian-buster, debian-bullseye, debian-bookworm, debian-sid, linuxmint-18, linuxmint-19, linuxmint-19.3, linuxmint-20.1, linuxmint-20.2. linuxmint-20.3, fedora-26, fedora-27, fedora-28, fedora-29, fedora-30, fedora-31, fedora-32, fedora-33, fedora-34, fedora-35, fedora-36,  centos-7-gcc_spkg, centos-stream-8, gentoo-python3.9, gentoo-python3.10, archlinux-latest, opensuse-15.3, opensuse-tumbleweed, slackware-14.2, conda-forge, ubuntu-bionic-i386, manylinux-2_24-i686, debian-buster-i386, centos-7-i386-gcc_spkg]
+        tox_system_factor: [gitpod, ubuntu-trusty-toolchain-gcc_9, ubuntu-xenial, ubuntu-bionic, ubuntu-focal, ubuntu-hirsute, ubuntu-impish, ubuntu-jammy,  debian-stretch, debian-buster, debian-bullseye, debian-bookworm, debian-sid, linuxmint-19, linuxmint-19.3, linuxmint-20.1, linuxmint-20.2, linuxmint-20.3, fedora-26, fedora-27, fedora-28, fedora-29, fedora-30, fedora-31, fedora-32, fedora-33, fedora-34, fedora-35, fedora-36, centos-7-devtoolset-gcc_11, centos-stream-8, gentoo-python3.9, gentoo-python3.10, archlinux-latest, opensuse-15.3, opensuse-tumbleweed, conda-forge, ubuntu-bionic-i386, manylinux-2_24-i686, debian-buster-i386, centos-7-i386-devtoolset-gcc_11]
         tox_packages_factor: [maximal]
         targets_pattern: [0-g, h-o, p, q-z]
     env:

.github/workflows/tox-optional.yml

@@ -38,10 +38,7 @@ jobs:
       fail-fast: false
       max-parallel: 6
       matrix:
-        # This list is different from the one in tox.yml:
-        # Trac #31526 switches gcc 4.x-based distributions to using the gcc_spkg configuration factor
-        # Trac #32281 removes gcc 4.x-based distributions whose binutils are unusable
-        tox_system_factor: [ubuntu-xenial, ubuntu-bionic, ubuntu-focal, ubuntu-hirsute, ubuntu-impish, ubuntu-jammy, debian-stretch, debian-buster, debian-bullseye, debian-bookworm, debian-sid, linuxmint-18, linuxmint-19, linuxmint-19.3, linuxmint-20.1, linuxmint-20.2. linuxmint-20.3, fedora-26, fedora-27, fedora-28, fedora-29, fedora-30, fedora-31, fedora-32, fedora-33, fedora-34, fedora-35, fedora-36,  centos-7-gcc_spkg, centos-stream-8, gentoo-python3.9, gentoo-python3.10, archlinux-latest, opensuse-15.3, opensuse-tumbleweed, slackware-14.2, conda-forge, ubuntu-bionic-i386, manylinux-2_24-i686, debian-buster-i386, centos-7-i386-gcc_spkg]
+        tox_system_factor: [ubuntu-trusty-toolchain-gcc_9, ubuntu-xenial, ubuntu-bionic, ubuntu-focal, ubuntu-hirsute, ubuntu-impish, ubuntu-jammy, debian-stretch, debian-buster, debian-bullseye, debian-bookworm, debian-sid, linuxmint-19, linuxmint-19.3, linuxmint-20.1, linuxmint-20.2. linuxmint-20.3, fedora-26, fedora-27, fedora-28, fedora-29, fedora-30, fedora-31, fedora-32, fedora-33, fedora-34, fedora-35, fedora-36, centos-7-devtoolset-gcc_11, centos-stream-8, gentoo-python3.9, gentoo-python3.10, archlinux-latest, opensuse-15.3, opensuse-tumbleweed, slackware-14.2, conda-forge, ubuntu-bionic-i386, manylinux-2_24-i686, debian-buster-i386, centos-7-i386-devtoolset-gcc_11]
         tox_packages_factor: [maximal]
         targets_pattern: [0-g, h-o, p, q-z]
     env:

.github/workflows/tox.yml

@@ -38,7 +38,7 @@ jobs:
       fail-fast: false
       max-parallel: 20
       matrix:
-        tox_system_factor: [gitpod, ubuntu-trusty, ubuntu-xenial, ubuntu-bionic, ubuntu-focal, ubuntu-hirsute, ubuntu-impish, ubuntu-jammy,  debian-stretch, debian-buster, debian-bullseye, debian-bookworm, debian-sid, linuxmint-17, linuxmint-18, linuxmint-19, linuxmint-19.3, linuxmint-20.1, linuxmint-20.2, linuxmint-20.3, fedora-26, fedora-27, fedora-28, fedora-29, fedora-30, fedora-31, fedora-32, fedora-33, fedora-34, fedora-35, fedora-36, centos-7, centos-stream-8, gentoo-python3.9, gentoo-python3.10, archlinux-latest, opensuse-15.3, opensuse-tumbleweed, slackware-14.2, conda-forge, ubuntu-bionic-i386, manylinux-2_24-i686, debian-buster-i386, centos-7-i386]
+        tox_system_factor: [gitpod, ubuntu-trusty-toolchain-gcc_9, ubuntu-xenial, ubuntu-bionic, ubuntu-focal, ubuntu-hirsute, ubuntu-impish, ubuntu-jammy,  debian-stretch, debian-buster, debian-bullseye, debian-bookworm, debian-sid, linuxmint-19, linuxmint-19.3, linuxmint-20.1, linuxmint-20.2, linuxmint-20.3, fedora-26, fedora-27, fedora-28, fedora-29, fedora-30, fedora-31, fedora-32, fedora-33, fedora-34, fedora-35, fedora-36, centos-7-devtoolset-gcc_11, centos-stream-8, gentoo-python3.9, gentoo-python3.10, archlinux-latest, opensuse-15.3, opensuse-tumbleweed, conda-forge, ubuntu-bionic-i386, manylinux-2_24-i686, debian-buster-i386, centos-7-i386-devtoolset-gcc_11]
         tox_packages_factor: [minimal, standard]
     env:
       TOX_ENV: docker-${{ matrix.tox_system_factor }}-${{ matrix.tox_packages_factor }}