UX for constraining build-time and runtime dependencies

There is a practical UX problem with `pyproject.toml` that we need to solve. A short description would be:
- Any project that builds against the `numpy` C API has to deal with API and ABI compatibility (see [the numpy docs on that](https://numpy.org/devdocs/user/depending_on_numpy.html?highlight=abi#adding-a-dependency-on-numpy) for details).
- as a consequence, the dependency specification for local development is different from the one needed for uploading an sdist to PyPI:
- for local development, you want loose dependencies, e.g. `numpy >= 1.19.5` (both a build and runtime dep)
- in sdist released to PyPI you want: `oldest-supported-numpy` (build dep metapackage, contains `==` pins per platform) and `numpy>='buildtime_dep,<1.25.0' # should be `N+3` where `N is current minor version.

That is obviously a complex set of dependencies to express in `pyproject.toml`, and we can't have two sets in there at the same time. The solution @FFY00 and I just discussed is:

1. On the `main` branch, use the loose development dependencies (`numpy>=1.19.5`)
2. In the CI job(s) that produce artifacts for release to PyPI, ensure that the _lowest_ supported dependencies (which may be platform-dependent) are installed.
3. In the _release_ branch, replace the dependencies with the build-time deps needed for uploading to PyPI.
4. Have support in `mesonpy` for the various kinds of pinnings people need for releases and API/ABI compatibility.

A rough sketch of how (4) could look:

```toml
[build-system]
requires = [
"numpy >= 1.19.5", # replace with (e.g.) `oldest-supported-numpy` in a release branch
]

[tool.mesonpy]
runtime_pinnings = [
# if build version detected is 1.20.3, replace with `>=1.20.3` (while preserving the upper bound,
# so end result would be for example `'>=1.20.3,<1.25.0')
"numpy = 'pin_compatible'",
"pythran = 'bugfix_only'", # if build version detected is 0.10.0, override the build dependency with `'>=0.10.0,<0.11.0'`
"torch = 'exact'", # runtime version must equal build-time version (PyTorch has no ABI stability guarantees)
]
```

Note that this feature is needed for some other packages than just NumPy too (basically every package offering a C/C++ API has to deal with this), but NumPy alone is important enough - every package containing even a single Cython extension which uses NumPy has to deal with this. _However_, it's probably still a bit too specific to want native support in `pyproject.toml` via a standard that all tools must implement. Hence the choice to put this in `mesonpy`.

Beginne damit, die vorgeschlagene Konfiguration in pyproject.toml und die im Issue beschriebene mesonpy-Unterstützung zu prüfen. Verfolge, wie sich Build-Zeit-Abhängigkeiten, runtime_pinnings und Abhängigkeitsänderungen im Release-Branch gegenseitig auswirken würden; im Bericht werden keine Quelldateien oder Tests genannt. Als abgeschlossen gilt die Definition und Implementierung des Pinning-Verhaltens für NumPy, nur Bugfix-Versionen und exakte Laufzeitabhängigkeiten einschließlich der Abdeckung von Release-Builds.

Vom Indexierungsmodell aus dem Issue-Text verfasst.