[CI] Pin cython version to fix cython compilation (#15353)

Cython 3.0.0 was recently released, but it is incompatible with the
current .pxi definitions in python/tvm/_ffi/_python. Pinning
the cython version until a working fix is created.

Author: lhutton1

docker/install/ubuntu_install_python_package.sh

@@ -25,7 +25,7 @@ pip3 install --upgrade \
     "Pygments>=2.4.0" \
     attrs \
     cloudpickle \
-    cython \
+    cython==0.29.34 \
     decorator \
     mypy \
     numpy==1.21.* \