Resolve "migrate to python 3.7"

• Migrate to python 3
• Require python 3.7, web2py 2.18.3, matplotlib 3.1.1, 0.25.2 or above
• Migrate to building utilities w2pext/utilities version 1
• Improve error reporting in edit_insert
• Few bugs fix here and there
• Closes #94 (closed)