Python 3 upgrades

[mwtoews]

This was mostly automated using pyupgrade, which does several things mostly related to Python 2 to 3 upgrades:

• The default encoding for Python 3 is utf-8 (PEP 3120), so no need to specify this
• Remove old from __future__ imports
• Use yield from statement (PEP 380)
• Use OSError instead of the IOError alias (PEP 3151)
• The default second argument to open() is "r", so this is removed. This one could be controversial, but the contexts where this is done it is clear the file is being read.

The only manual edit is to rephrase an exception message in src/rasterstats/main.py

[perrygeo]

@mwtoews Looks good, thanks once again!