Pin dependencies to resolve several CVEs

[bachya]

Describe what the PR does:

SSIA.

Does this fix a specific issue?

Fixes https://github.com/bachya/pytile/security/dependabot/5 Fixes https://github.com/bachya/pytile/security/dependabot/7 Fixes https://github.com/bachya/pytile/security/dependabot/8

Checklist:

• [ ] Confirm that one or more new tests are written for the new functionality.
• [x] Run tests and ensure everything passes (with 100% test coverage).
• [ ] Update README.md with any new documentation.

[codeclimate[bot]]

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