from_disk: Fix mypy error with dulwich >= 0.20.13

dulwich recently adds PEP-561 compatibility [1] so ignore typecheck for older dulwich versions.

• [1] https://github.com/dulwich/dulwich/commit/e116a6eb05598eb59c3e3d2239f6fcd13a1e25e3

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Migrated from D4557 (view on Phabricator)