CI: drop Pyston from test matrix

Pyston is unmaintained. The most recent Pyston packages are only
available for for Ubuntu 20.04. GitHub Actions is dropping support for
this distribution. With this it is becoming more demanding to test
this interpreter. The latest Pyston release support Python 3.8, which
is EOL for CPython. meson-python does not have any Pyston specific
code. Maintaining test coverage for this interpreter does not seem
worth the effort that it would require.

Author: dnicolodi

.github/workflows/tests.yml

@@ -252,38 +252,6 @@ jobs:
         run: python -m pytest --showlocals -vv
         shell: C:\cygwin\bin\env.exe CYGWIN_NOWINPATH=1 CHERE_INVOKING=1 C:\cygwin\bin\bash.exe -leo pipefail -o igncr {0}
 
-  pyston:
-    runs-on: ubuntu-20.04
-    strategy:
-      fail-fast: false
-      matrix:
-        python:
-          - '3.8'
-        meson:
-          -
-
-    steps:
-      - name: Checkout
-        uses: actions/checkout@v4
-
-      - name: Install pyston
-        run: |
-          wget https://github.com/pyston/pyston/releases/download/pyston_2.3.5/pyston_2.3.5_20.04_amd64.deb
-          sudo apt install $(pwd)/pyston_2.3.5_20.04_amd64.deb
-
-      - name: Install Ninja
-        run: sudo apt-get install ninja-build
-
-      - name: Install Meson
-        run: python -m pip install "meson ${{ matrix.meson }}"
-        if: ${{ matrix.meson }}
-
-      - name: Install
-        run: pyston -m pip install .[test]
-
-      - name: Run tests
-        run: pyston -m pytest --showlocals -vv
-
   homebrew:
     runs-on: macos-latest
     strategy: