Clean up imports of old modules

[sveinse]

The canopen package have import leftovers from earlier versions of Python. This PR cleans up the no longer valid imports.

• import Queue dates to py 2
• import collections.*Mapping was changed to collections.abc.*Mapping in py 3.3
• import ConfigParser dates to py 2
• import xml.etree.cElementTree is deprecated since py 3.3. It will pick the fastest option when using xml.etree.ElementTree
• Fixed .base into canopen.pdo.base in canopen/pdo/init.py
• Fixed .base into canopen.sdo.base in canopen/sdo/init.py

[acolomb]

Looks good, but I'd like some discussion / reasoning about using absolute imports within the same package. DRY tells us to leave out the package name in places within the same package, and I've seen very few use cases where relative imports don't work.

[acolomb]

Why change to absolute package reference here? I'd actually rather go the other way and use relative imports within the same package. Maybe it's a matter of taste, but I liked the old style.

[sveinse]

No other reason that PEP 8 recommends using absolute imports, so it's become a habit for many projects. PEP 8 also states "explicit relative imports are an acceptable alternative to absolute imports, especially when dealing with complex package layouts where using absolute imports would be unnecessarily verbose". So either is acceptable, but with preference to the former. And lastly, the rest of canopen is using absolute imports. I find it messy that the import styles are mixed.

[acolomb]

Same here.