Glyphslib

Fixed 'ImportError: no module named interpolation' in fontmake
googlei18n/fontmake451

* Remove interpolation capabilities and thereby MutatorMath dependency. Use fontmake instead.
* Repurposed and extended glyphs2ufo and ufo2glyphs command-line scripts to round-trip between formats with as much metadata intact as possible.
* Fix a roundtrip bug where the Regular would be renamed to Normal

- Added support for "Reencode Glyphs" instance custom parameter (378, 205).

- Allow to load `GSFont` from a `os.PathLike` object on python3 (359).
- Fixed parsing and roundtripping of color tuples (363).
- Fixed regression in the generated `designspace.filename` when a custom `family_name` is provided that could lead to fontmake outputs overwriting each other (377).

Added:

- `glyphsLib.to_glyphs(DesignSpaceDocument | list<defcon.Font>) -> GSFont` has been implemented and can create a Glyphs font from either a designspace document + source UFOs, or from a list of UFOs.
- `glyphsLib.to_designspace(GSFont) -> DesignSpaceDocument` can translate a Glyphs font into a designspace document and UFOs (reachable via `document.sources[i].font`)
- `to_ufos()` / `to_designspace()` (resp. `to_glyphs()`) have a new parameter `minimize_glyphs_diffs` (resp. `minimize_ufo_diffs`) which can be set to True when round-tripping. When true, glyphsLib stores more information in the produced file, in order to minimize diffs when coming back to the original file. It's interesting when using glyphsLib on files that are under version control.
- Handle `Axes` and masters' `Axis Location` custom parameters.
- Handle Glyphs "annotations" (under a lib key in generated UFOs).
- Handle background images.
- Handle smart components axes and values (but NOT outlines: there is no interpolation of smart components yet).
- Handle multiple unicode values per glyph.
- Handle guidelines.
- Handle Glyphs hints (under a lib key in generated UFOs).
- Support Glyphs API methods: `GSFont.kerningForPair`, `GSFont.setKerningForPair`, `GSFont.removeKerningForPair`

Fixed:

- Don't crash on references to components that don't exist
- Parse various formats for multiple unicode values.
- Parse binary data as hex-encoded bytes.

Changed:

- The command line argument `-R/--no-round` is now `-r/--round-instances` and does the opposite.
- The default names for the width and weight axes is now "Width" and "Weight" in the generated designspace documents.

- Write the `grid` master property as `gridLength` in the *.glyphs file. Make `gridLength` a read-only property of `GSMaster` (equal to `master.grid/master.gridSubDivisions`) (298).
- Allow parsing datetime string without explicit timezone (fa9cdc8).
- [MetaTools] Do not fetch license info when using external xml (296).