ci: unpin python, work around mamba bug (#4293)

- Unpin Python 3.12.
- Lowercase the Cython requirement
(mamba-org/mamba#4251)

Author: lidavidm

ci/conda_env_python.txt

@@ -15,7 +15,7 @@
 # specific language governing permissions and limitations
 # under the License.
 
-Cython
+cython
 importlib-resources
 # libxml2 broke ABI compatibility
 # https://github.com/conda-forge/arrow-cpp-feedstock/issues/1740

dev/release/verify-release-candidate.sh

@@ -569,8 +569,7 @@ test_python() {
 
   # Build and test Python
   maybe_setup_virtualenv cython duckdb pandas polars protobuf pyarrow pytest setuptools_scm setuptools importlib_resources || exit 1
-  # XXX: pin Python for now since various other packages haven't caught up
-  maybe_setup_conda --file "${ADBC_DIR}/ci/conda_env_python.txt" python=3.12 || exit 1
+  maybe_setup_conda --file "${ADBC_DIR}/ci/conda_env_python.txt" || exit 1
 
   if [ "${USE_CONDA}" -gt 0 ]; then
     CMAKE_PREFIX_PATH="${CONDA_BACKUP_CMAKE_PREFIX_PATH}:${CMAKE_PREFIX_PATH}"