Resolve "migrate to python 3.7"

• Migrate to python 3
• Require python 3.7, web2py 2.18.3, matplotlib 3.1.1, pandas 0.25.2 or above.
• Require plugin_dbui 1.0, plugin_event 1.0 or above
• Update to building tools w2pext/utilities release 1.0
• Add a module pages_manager
• Closes #4 (closed)