CI Linux: trying to install tkinter_gl from wheel because its buildin…

…g requires setuptools 60.0 and thus doesn't work with old pythons.

.github/workflows/linux.yml

@@ -31,7 +31,7 @@ jobs:
             pip install git+https://github.com/3-manifolds/PLink &&
             pip install git+https://github.com/3-manifolds/snappy_manifolds &&
             pip install git+https://github.com/3-manifolds/Spherogram &&
-            pip install git+https://github.com/3-manifolds/tkinter_gl
+            pip install https://files.pythonhosted.org/packages/1d/b8/a2e2979750527f291d6e181172e08495ebb65374ee7fbfbfc4fe33094532/tkinter_gl-1.0a2-py3-none-any.whl
           CIBW_REPAIR_WHEEL_COMMAND_LINUX: auditwheel repair --strip -w {dest_dir} {wheel}
           CIBW_TEST_COMMAND: python -m snappy.test --skip-modern-opengl