build: Update python_requires to Python 3.8

* As numpy >=1.22.0 is required, and that support Python 3.8+, update
  python_requires to Python 3.8.
   - c.f. https://pypi.org/project/numpy/1.22.0/

Author: matthewfeickert

setup.py

@@ -128,7 +128,7 @@ def compile():
                            'runSModelS.py=smodels.tools.runSModelS:main',
                            'smodelsTools.py=smodels.tools.smodelsTools:main' ]
     },
-    python_requires='>=3.7',
+    python_requires='>=3.8',
     install_requires=requirements(),
     data_files=dataFiles() ,
     description=("A tool for interpreting simplified-model results from the "