[mlir] Relax version requirement for PyYAML in mlir

Some Ubuntu 20.04 images come with PyYAML 5.3.1 pre-installed through distutils. This makes pip very angry. See https://github.com/yaml/pyyaml/issues/349.

Since older versions of PyYAML should work for mlir, relax the version requirement to ease developer setup.

Reviewed By: stellaraccident

Differential Revision: https://reviews.llvm.org/D143523