Fix python_requires versioning for new setuptools

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

Patch by Justice Adams!

Author: azharudd

setup.py

@@ -138,5 +138,5 @@
 
     ext_modules=[cPerf],
 
-    python_requires='>=3.6.*',
+    python_requires='>=3.6',
 )