Iris-grib release v0.20.0

This is a minor release of iris-grib, which is available on conda-forge and PyPI.

New Features

Save LAEA grid by @abooton in #343
pdt6 & gdt140 take three by @pp-mo in #401
Grib1 loading defines a GRIB_PARAM attribute, like grib2 load. by @pp-mo in #402
Save GDT20 - Polar Stereographic - take 2 by @trexfeathers in #405

add iris-sample-data test dependency by @bjlittle in #413
add pytest-mock dependency by @bjlittle in #424
nep29: drop py39 and support py312 by @bjlittle in #453
(Iris >= 3.10) Provide masked/unmasked meta for calls to 'as_lazy_data' by @pp-mo in #463
Eccodes 2v36 by @pp-mo in #504

Beyond new features, the v0.20.0 release was mostly focussed on a work program to update the repository management.
The Scientific Python development guidelines was used as a guide for this, and its automated repo checks are now implemented in the project pre-commit.

build system
- add pyproject.toml and remove setup.py by @HGWright in #408
- Adding setuptools_scm to iris grib for automatic versioning by @HGWright in #444
repo checks
- Configure for mypy by @stephenworsley in #407
- Ruff adopt by @HGWright in #430
- add dependabot by @bjlittle in #426
- Added in linkchecks and coverage checks for gha ci by @ESadek-MO in #432
- update dependencies and enable codecov by @bjlittle in #454
- add code-of-conduct by @bjlittle in #460
- Migrate iris_grib to src/iris_grib by @bjlittle in #450
- ignore noisy pull-requests with .git-blame-ignore-revs by @bjlittle in #452
- adopt repo-review pre-commit hook by @bjlittle in #456
- add codecov configuration by @bjlittle in #459
- tests:
  - Convert tests and doctests from Cirrus to GHA by @ESadek-MO in #415
  - Migrate to pytest by @bjlittle in #420
- developer
  - Add pre-commit config by @trexfeathers in #400

Full Changelog: v0.19.1...v0.20.0