Update minimum Python version required

[nightlark]

• Put together a list of simulators we need to support (@phlptp)
• Find out what versions of Python are currently supported by vendors of simulation software HELICS may need to work with

[nightlark]

• PSS/E (3.7 on version 34 or 3.9 on version 35)
• CYME
• GridApps-D (3.10)

Likely going to try to follow https://devguide.python.org/versions/ -- if someone is using an older simulator that doesn't support a non-EOL Python 3.x version, they can use an older pyhelics version (and just miss out on some of the newer HELICS 3.5+ features).