Uharfbuzz

- Added two new Buffer methods to set the script and language from OT tags: `buf.set_script_from_ot_tag(tag)` and `buf.set_language_from_ot_tag(tag)`

- Fixed hardcoded limit for retrieving feature, script and language tags (32, fixes 31).
- Exposed HarfBuzz "message" api, via `buf.set_message_func(func)` (27).
- Exposed HarfBuzz api for retrieving baseline values from the `BASE` table, as `uharfbuzz.ot_layout_get_baseline(...)` (30)

Added callback support for vertical advance and vertical origin (26).

- Return False when nominal glyph callback returns '.notdef' (i.e. the character is not supported by the font) (24, fixes 23).
- Updated embedded HarfBuzz library to 2.6.4
- Added wheels for Python 3.8.

- Fixed issue with `Face.create_for_tables` (17).
- Fixed building on macOS 10.14 where `libstc++` is no longer available (16).
MacOS wheels now require 10.9 or above, because they are linked with the new `libc++` targeting C++11.
- Updated embedded HarfBuzz library to v2.4.0:

- Initialize Buffer, Face, Font, FontFuncs classes in their default `__cinit__` constructor; deprecate alternative `create()` constructors (12).
- Avoid to copy unicode strings when populating the buffer, pass directly to hb_buffer_add_* their inner data pointers from the PEP393 canonical or shortest-representation form (13).
- Set up test suite with support for test coverage of Cython files (14).
- Explicitly require Python >= 3.5 in setup.py. This is required since we use PEP393 flexible unicode strings API, the typing module, etc.