Replace deprecated pkg_resources

[hcho3]

pkg_resources\__init__.py:121: DeprecationWarning: pkg_resources is deprecated as an API
    warnings.warn("pkg_resources is deprecated as an API", DeprecationWarning)

[codecov[bot]]

Codecov Report

Patch coverage: 50.00% and project coverage change: -0.64 :warning:

Comparison is base (46b194f) 85.31% compared to head (5c577a5) 84.68%.

:exclamation: Current head 5c577a5 differs from pull request most recent head cc6a100. Consider uploading reports for the commit cc6a100 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## mainline #486 +/- ## ============================================== - Coverage 85.31% 84.68% -0.64% Complexity 42 42 ============================================== Files 112 111 -1 Lines 9910 8849 -1061 Branches 617 40 -577 ============================================== - Hits 8455 7494 -961 + Misses 1432 1332 -100 Partials 23 23 ``` | [Impacted Files](https://codecov.io/gh/dmlc/treelite/pull/486?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dmlc) | Coverage Δ | | |---|---|---| | [python/treelite/contrib/msvc.py](https://codecov.io/gh/dmlc/treelite/pull/486?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dmlc#diff-cHl0aG9uL3RyZWVsaXRlL2NvbnRyaWIvbXN2Yy5weQ==) | `0.00% <0.00%> (-68.12%)` | :arrow_down: | | [python/treelite/frontend.py](https://codecov.io/gh/dmlc/treelite/pull/486?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dmlc#diff-cHl0aG9uL3RyZWVsaXRlL2Zyb250ZW5kLnB5) | `77.89% <100.00%> (ø)` | | ... and [67 files with indirect coverage changes](https://codecov.io/gh/dmlc/treelite/pull/486/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dmlc)

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