In 89a7e19, an API for converting "dvi glyph indices" (as stored
in a dvi file) to FreeType-compatible keys (either "indices into
the native charmap" or "glyph names") was introduced.  It was
intended that end users (i.e., backends) would check the type of
`text.glyph_name_or_index` ((A) int or (B) str) and load the glyph
accordingly ((A) `FT_Set_Charmap(native_cmap); FT_Load_Char(index);` or
(B) `FT_Load_Glyph(FT_Get_Name_Index(name));`); however, with the future
introduction of {xe,lua}tex support, this kind of type checking becomes
inconvenient, because {xe,lua}tex's "dvi glyph indices", which are
directly equal to FreeType glyph indices (i.e. they would be loaded with
`FT_Load_Glyph(index);`), would normally also be converted to ints.

This PR introduces a new API (`Text.index`) that performs this mapping
(via the new `DviFont._index_dvi_to_freetype`), always mapping to
FreeType glyph indices (i.e. one can always just call `FT_Load_Glyph`
on the result).  To do so, in case (A) it loads itself the native
charmap (something the end user needed to do by themselves previously)
and performs the cmap-to-index conversion (`FT_Get_Char_Index`)
previously implicit in `FT_Load_Char`; in case (B) it performs itself
the name-to-index conversion (`FT_Get_Name_Index`).  When {xe,lua}tex
support is introduced in the future, `_index_dvi_to_freetype` will
just return the index as is.

The old APIs are not deprecated yet, as other changes will occur for
{xe,lua}tex support (e.g. font_effects will go away and be replaced by
`.font.effects`, as {xe,lua}tex don't store that info in the pdftexmap
entry), and grouping all API changes together seems nicer (to only
require a single adaptation step by the API consumer).

Expire deprecation of nth_coord arguments

Remove deprecated get_tick_iterator()

The combination of matplotlib#30014 and matplotlib#30015 made this import redundant, but as
each PR was indepenently tested and reviewed, nothing was evident until
both were merged.

Remove unused `_api` import

This patch speeds up conversions of `#rgba`-type formats by between 25%
and 40% (while shortening the implementation), although real benefits
should be limited because of caching in to_rgba.

They are clearly intended for internal use, and we may want to change
their internal representation in the future to support xetex/luatex
(which expose more font types).

Rework mapping of dvi glyph indices to freetype indices.

Make PdfFile font-related attributes private.

Micro-optimize _to_rgba_no_colorcycle.

…meaning

…-diagram

Update diagram in subplots_adjust documentation to clarify parameters

Enable linting of .pyi files

Version 0.18 should restore handling of symlinks:
mesonbuild/meson-python#728