This improves packaging reliability because it prevents tests from
running against the current directory instead of the installed version
of the package.

- Add `pyproject.toml` and gather configuration there. Move `.ruff.toml`
  contents into `pyproject.toml`.
- Add `requirements-test.txt`.

Add type-annotations broadly, but prioritizing interface objects and
methods. Adjust text-width to 100, remove Python 2 support, etc.

scanny#943

Remove mention of a `Px` subtype of `Length` since no such subtype
exists. Turns out a pixel has various sizes on different systems so no
standard unit of measure is possible.

scanny#972

Naive "max + 1" slide-id allocation algorithm assumed that slide-ids
were assigned from bottom up. Apparently some client assigns slide ids
from the top (2,147,483,647) down and the naive algorithm would assign
an invalid slide-id in that case.

Detect when the assigned id is out-of-range and fall-back to a robust
algorithm for assigning a valid id based on a "first unused starting at
bottom" policy.

Accommodate system dates before 1980-01-01.

This should have no effect for users with "normal" system clocks but
allows the package to save files in the unusual case the system clock is
set to a date prior to 1980.

At least one client, perhaps Adobe PDF Converter, produces a PPTX with
JPEG images mapped to the non-existent `image/jpg` MIME-type rather than
the correct `image/jpeg`.

Accept `image/jpg` as an alias for `image/jpeg`, which is consistent
with the behavior of PowerPoint which loads these files without
complaint.

- Move to `pyproject.toml`
- Update support to 3.8+

Indicate availability of type annotations to type-checkers.

Some enum values like `MSO_SHAPE_TYPE.MIXED` are never produced by
`python-pptx` and so were removed during the enum modernization in
commit `01b86e64`. However, in at least one downstream system these
values are referenced even though they can never actually occur.

Replace these "read-only" values to avoid downstream breakage.