[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 yaml/pyyaml#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

Author: sstamenova

mlir/python/requirements.txt

@@ -1,4 +1,4 @@
 numpy>=1.19.5, <=1.23.5
 pybind11>=2.8.0, <=2.10.3
-PyYAML==6.0
+PyYAML>= 5.3.1, <=6.0
 dataclasses>=0.6, <=0.8