ci: fix ddtrace version detection

[brettlangdon]

What does this PR do?

PR #763 introduces removing all patch.py files from ddtrace to help reduce layer size. However, the version detection logic we had was not working.

This change fixes that so we properly call the right .py file branch in Dockerfile when ddtrace version >=4.7.0rc3

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Types of Changes

• Bug fix
• New feature
• Breaking change
• Misc (docs, refactoring, dependency upgrade, etc.)

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