Fix python_requires versioning for new setuptools

[justice_adams]

Summary

Newer versions of setuptools will fail to install LNT with the following error:

× python setup.py egg_info did not run successfully.
  │ exit code: 1
  ╰─> [1 lines of output]
      error in LNT setup command: 'python_requires' must be a string containing valid version specifiers; Invalid specifier: '>=3.6.*'
      [end of output]

From reading the documentation, I believe it was a bug to allow >=3.6.* previously, as I don't think wild cards are supported here
https://peps.python.org/pep-0345/#requires-python

[cmatthews]

LGTM. Thanks!

[justice_adams]

Merged in with 0476dede37838e2f765f50dd26734cfced9d90d9

[justice_adams]

https://github.com/llvm/llvm-lnt/commit/0476dede37838e2f765f50dd26734cfced9d90d9

setup.py

install_requires=[
         "pyyaml==5.1.2",
         "requests",
         "lit==0.11.1",
         "certifi"
     ],
 
     ext_modules=[cPerf],
 
-    python_requires='>=3.6.*',
+    python_requires='>=3.6',
 )