Python 3 support for SLING API

[ringgaard]

Due to the end-of-life for Python 2, we are moving the SLING project to Python 3. Since Python 2.7 will no longer be maintained after January 1, 2020, including no security updates, we will not support Python 2.7 going forward.

I have migrated the pyapi library to use Python 3 and updated the Python modules in the SLING wheel accordingly.

There are still some parts that need to be migrated, notably the parser trainer (sling/nlp/parser/trainer), the wikibot (python/wikibot) and the category browser (sling/nlp/wikicat).

One major change form Python 2 to 3 is that strings are now Unicode and the new API supports Unicode in all appropriate places. However, this introduces some subtle ambiguities where we used strings for both binary and text.

Strings in the frames stores are returned as Unicode strings in the Python API, but this can lead to UnicodeEncodeError exceptions for invalid UTF8 strings. You can now use the get(role, binary=True) method to get the value of a string in a frame store as bytes.

The key and value returned from a record file are returned as bytes, so you have to manually convert these to strings, e.g. key.decode(), if you need these as strings. You can use both string and bytes when writing to record files.

Another subtle issues is the Document.text property. The indices into the document text, e.g. Token.begin and Token.end, are offsets into the UTF-8 encoded document text. To prevent any UTF8/Unicode round trip conversion issues, I have kept Document.text as bytes.

We use Python 3.5 as the default Python version for now, but the code seems to work for Python 3.6 and 3.7 as well. You can set PYVER in sling/pyapi/BUILD to 35, 36 or 37 to change the Python runtime version used for building.

I have made a setup.sh script to make it easier to set up the SLING development environment. Please run this to get your SLING environment updated to the new Python3-based setup.

[anders-sandholm]

Thanks for upgrading the code to Python3. LGTM. Looking fwd to getting all of SLING upgraded to Python3.

[anders-sandholm]

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