Download PDFs, source tarballs from export.arxiv.org (lukasschwab#172)

Per arXiv documentation on preferred usage. Closes lukasschwab#87.

+ Change range of supported Python versions: 3.9...3.13 (drop 3.7, 3.9)
  See https://devguide.python.org/versions/
+ Rework CI so it runs: bump action versions, consolidate ruff steps
+ Bump version to 2.2.0 for release

Co-authored-by: tobiaswuerth <[email protected]>
Co-authored-by: Lukas Schwab <[email protected]>

Use `Optional` type hints where appropriate (lukasschwab#168)

+ Belatedly `make format`
+ Convenience: bump version to 2.1.3 for release

Bump version to 2.1.2 for release (lukasschwab#166)

Bump version to 2.1.1 for release (lukasschwab#164)

Bump version to 2.1.0 (lukasschwab#146)

Bump version to 2.0.0 (lukasschwab#139)

+ Improve `max_results`/`delay_seconds` types, defaults (lukasschwab#138)
+ Eliminate `get`, deprecate `Search.Results` (lukasschwab#137)
+ Accelerate CI integration tests (lukasschwab#140)

Replace `/` with `_` in legacy-format IDs in download filenames (luka…

…sschwab#118)

Fixes lukasschwab#117. Differentiates character-escaping strategies for paper
titles and paper IDs:

+ IDs: only replace `/` with `_`, to account for legacy-form IDs.
+ Titles: replace non-word (i.e. `[^\w]`) characters with `_`.

Adds a regression test.

Indicate Python version in trove classifiers (lukasschwab#112)

Concession to get the Shields.io badge to work. `python_requires`
doesn't cut it: badges/shields#5550

Increment patch version to allow PyPI release: 1.4.7.

Set python_requires >=3.7 (lukasschwab#111)

Should capture what we test in CI: 3.7, ..., 3.10, 3.11. Increment the
version to 1.4.6 to reflect a patch: this isn't a breaking change for
any of the supported versions.

Update README.md version badge to resolve supported versions from PyPI.

Disambiguate generic `list` type hints (lukasschwab#109)

* Disambiguate generic list types

Also updates string annotations in constructor arguments.

Imports `annotations` from `__future__`, which allows referring to
class types before they're defined.

Verification: searched for all instances of `: list`; there aren't any
left in arxiv.py. Generated docs look good.

* Increment version for patch