Replace IOError and WindowsError aliases with OSError

[hugovk]

Fixes #4535.

Changes proposed in this pull request:

• Changed in [Python] version 3.3: EnvironmentError, IOError, WindowsError, socket.error, select.error and mmap.error have been merged into OSError, and the constructor may return a subclass.

• https://docs.python.org/3/library/exceptions.html#OSError
• pyupgrade replaced some in Drop support for EOL Python 2.7 #4109
• This PR replaces the rest

Two questions:

In src/PIL/ImageFile.py, this PR renames the raise_ioerror helper function to raise_oserror. Technically this could be considered a breaking change.

1. Should we keep the old raise_ioerror with a deprecation warning?
2. Should we name the new one _raise_oserror to indicate it is internal?