Make patch release 4.1.1

[hcho3]

• Restore support for Python 3.8
• PyPy compatibility patch
• Load libtreelite.so with symbols visible globally. This is useful when using TL2cgen or another library that depends on libtreelite.so.

[codecov[bot]]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (70b4206) 84.55% compared to head (82a79f8) 84.43%.

Files	Patch %	Lines
python/treelite/model.py	71.42%	4 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## release_4.1 #555 +/- ## =============================================== - Coverage 84.55% 84.43% -0.13% =============================================== Files 71 71 Lines 6171 6180 +9 Branches 518 518 =============================================== Hits 5218 5218 - Misses 953 962 +9 ```

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