[Fix] fix readthedocs building

[liuwenran]

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Motivation

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Modification

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BC-breaking (Optional)

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Use cases (Optional)

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Checklist

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Before PR:

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• Pre-commit or linting tools indicated in CONTRIBUTING.md are used to fix the potential lint issues.
• Bug fixes are covered by unit tests, the case that causes the bug should be added in the unit tests.
• New functionalities are covered by complete unit tests. If not, please add more unit test to ensure the correctness.
• The documentation has been modified accordingly, including docstring or example tutorials.

After PR:

• If the modification has potential influence on downstream or other related projects, this PR should be tested with some of those projects.
• CLA has been signed and all committers have signed the CLA in this PR.

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Files	Coverage Δ
mmagic/__init__.py	82.75% <100.00%> (ø)

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