Issues with conditional imports

[JukkaL]

Even though #649 was closed, there still remains several issues related to conditional imports. This issue is a top-level issue for these.

Related, more specific issues:

• Can't redefine name imported from another module #591
• Conditional class import #1107
• mypy bug with try/except conditional imports #1153
• Conditional import of izip is problematic #1026

[gvanrossum]

Also #1289.

[JukkaL]

Also #1327.

[JukkaL]

And #1334.