encode development requirements in pyproject.toml (#32616)

Add a `project` block to the toml config along with development and CI
dependencies and a minimal `build-system` block, doing basically
nothing, so that spack can be bootstrapped to a full development
environment with:

```shell
$ hatch -e dev shell
```

or for a minimal environment without hatch:

```shell
$ python3 -m venv venv
$ source venv/bin/activate
$ python3 -m pip install --upgrade pip
$ python3 -m pip install -e '.[dev]'
```

This means we can re-use the requirements list throughout the workflow
yaml files and otherwise maintain this list in *one place* rather than
several disparate ones.  We may be stuck with a couple more temporarily
to continue supporting python2.7, but aside from that it's less places
to get out of sync and a couple new bootstrap options.

Co-authored-by: Adam J. Stewart <ajstewart426@gmail.com>

lib/spack/spack/__init__.py

@@ -3,11 +3,20 @@
 #
 # SPDX-License-Identifier: (Apache-2.0 OR MIT)
 
-#: (major, minor, micro, dev release) tuple
-spack_version_info = (0, 19, 0, "dev0")
-
 #: PEP440 canonical <major>.<minor>.<micro>.<devN> string
-spack_version = ".".join(str(s) for s in spack_version_info)
+__version__ = "0.19.0.dev0"
+spack_version = __version__
+
+
+def __try_int(v):
+    try:
+        return int(v)
+    except ValueError:
+        return v
+
+
+#: (major, minor, micro, dev release) tuple
+spack_version_info = tuple([__try_int(v) for v in __version__.split(".")])
+
 
 __all__ = ["spack_version_info", "spack_version"]
-__version__ = spack_version