Improved import handling in Python

[maximiliankaul]

There is some support for handling import statements in Python. This issue is tracking open points with the goal to correctly handle imports in the end:

• Importing and using unknown variables results in a FieldDeclaration #1603
• relative imports using . and/or ..
• from math import pi and then using pi -> there is currently no edge between the two nodes connecting them
• Declaration AST placement #1602
• Importing a module and then overwriting the variable (import foo; foo=42; foo.bar) is currently not handled correctly.
• from ..foo.bar import a as b, c