Remove mypy from dependencies.

[dnoursi]

Description

It is inconvenient for us to upgrade our environment's labelbox version without upgrading our mypy version, because of this dependency. I think that it can be appropriate for mypy to be in dev-dependencies only. This change would help us to upgrade our labelbox sdk.

Type of change

Please delete options that are not relevant.

• [x ] Bug fix (non-breaking change which fixes an issue)

All Submissions

• [x ] Have you followed the guidelines in our Contributing document?
• [x ] Have you provided a description?
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New Feature Submissions

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• Have you added thorough tests for your new feature?
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Changes to Core Features

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[paulnoirel]

LGTM