This:

• adds UnicodeError and subtypes,
• makes TextIOWrapper throw a UnicodeDecodeError on malformed utf-8 in a file,
• makes TextIOWrapper writable if underlying buffer is writable,
• makes TextIOWrapper not readable if underlying buffer is writable.

When the wrapper is not readable/writable, CPython throws an io.UnsupportedOperation which inherits from both OSError and ValueError. I just throw ValueError here – do we have multiple inheritance yet to do a proper UnsupportedOperation?

Also, all the UnicodeError types in CPython have more internal structure than the simple exceptions I added here.

Some more things to be done to close the rest of #547 are:

• implement closing files, buffers, text wrappers,
• perhaps make file a separate PyObjectPayload to hide the underlying Rust File internally and no longer depend on fileno?,
• make fileno just an accessor method.

But I’d rather try to do them in separate pull requests (or let somebody else do them if anyone gets to it before me).