Add Py version 3.7 back

Author: ssolson

README.md

@@ -24,7 +24,7 @@ See the [MHKiT documentation](https://mhkit-software.github.io/MHKiT) for more i
 
 Installation
 ------------------------
-MHKiT-Python requires Python (3.8, or 3.9) along with several Python 
+MHKiT-Python requires Python (3.7, 3.8, or 3.9) along with several Python 
 package dependencies.  MHKiT-Python can be installed from PyPI using the command ``pip install mhkit``.
 See [installation instructions](https://mhkit-software.github.io/MHKiT/installation.html) for more information.