Fix pinning to treelite rather than lightgbm.

[benowen-bom]

Currently the scheduled tests are failing:

improver_tests/calibration/rainforests_calibration/conftest.py:299: ModuleNotFoundError
=========================== short test summary info ============================
ERROR improver_tests/calibration/rainforests_calibration/test_ApplyRainForestsCalibrationLightGBM.py::test__check_num_features[treelite] - ModuleNotFoundError: No module named 'treelite_runtime'
ERROR improver_tests/calibration/rainforests_calibration/test_ApplyRainForestsCalibrationLightGBM.py::test__empty_config_warning[treelite] - ModuleNotFoundError: No module named 'treelite_runtime'
ERROR improver_tests/calibration/rainforests_calibration/test_ApplyRainForestsCalibrationLightGBM.py::test__evaluate_probabilities[treelite] - ModuleNotFoundError: No module named 'treelite_runtime'
ERROR improver_tests/calibration/rainforests_calibration/test_ApplyRainForestsCalibrationLightGBM.py::test__calculate_threshold_probabilities[treelite] - ModuleNotFoundError: No module named 'treelite_runtime'
ERROR improver_tests/calibration/rainforests_calibration/test_ApplyRainForestsCalibrationLightGBM.py::test__get_ensemble_distributions[treelite] - ModuleNotFoundError: No module named 'treelite_runtime'
ERROR improver_tests/calibration/rainforests_calibration/test_ApplyRainForestsCalibrationLightGBM.py::test_lead_time_without_matching_model[treelite] - ModuleNotFoundError: No module named 'treelite_runtime'
ERROR improver_tests/calibration/rainforests_calibration/test_ApplyRainForestsCalibrationLightGBM.py::test_process_ensemble_specifying_thresholds[treelite] - ModuleNotFoundError: No module named 'treelite_runtime'
ERROR improver_tests/calibration/rainforests_calibration/test_ApplyRainForestsCalibrationLightGBM.py::test_process_deterministic[treelite] - ModuleNotFoundError: No module named 'treelite_runtime'
= 5060 passed, 545 skipped, 1 xfailed, 5601 warnings, 8 errors in 196.43s (0:03:16) =
Error: Process completed with exit code 1.

The issue seems to be a change within treelite package. Fix to pin this package with #1966 pinned lightgbm rather than treelite. This PR is to unpin lightgbm and pin treelite.

Testing:

• [x] Ran tests and they passed OK

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (fb4b29d) 98.40% compared to head (08d2acf) 98.40%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1981 +/- ## ======================================= Coverage 98.40% 98.40% ======================================= Files 124 124 Lines 12125 12125 ======================================= Hits 11931 11931 Misses 194 194 ```

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[benowen-bom]

Thanks @btrotta-bom.