Python 2 Cruft

[djhoese]

This issue describes the various code pieces that should be removed or modified when Python 2 support is eventually removed from SatPy in order to keep the code clean. It will likely be difficult to keep this issue up to date, but I figure it would be good to make an attempt. SatPy maintainers should update the bulleted list below. SatPy contributors (no issue editing permissions) should comment with code lines/blocks that need to modified and how.

• yaml_reader.py: Use of abstractproperty is deprecated in Python 3.3+. It should be replaced by @property followed by @abstractmethod: https://docs.python.org/3/library/abc.html#abc.abstractmethod

These changes don't all have to occur in one PR. They can't be merged until SatPy's Python 2.7 support is dropped which is will likely be some time shortly after numpy's support of it is dropped.