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

lib/spack/spack_installable/main.py

@@ -0,0 +1,59 @@
+import os
+import sys
+from os.path import dirname as dn
+
+
+def main(argv=None):
+    # Find spack's location and its prefix.
+    this_file = os.path.realpath(os.path.expanduser(__file__))
+    spack_prefix = dn(dn(dn(dn(this_file))))
+
+    # Allow spack libs to be imported in our scripts
+    spack_lib_path = os.path.join(spack_prefix, "lib", "spack")
+    sys.path.insert(0, spack_lib_path)
+
+    # Add external libs
+    spack_external_libs = os.path.join(spack_lib_path, "external")
+
+    if sys.version_info[:2] <= (2, 7):
+        sys.path.insert(0, os.path.join(spack_external_libs, "py2"))
+
+    sys.path.insert(0, spack_external_libs)
+    # Here we delete ruamel.yaml in case it has been already imported from site
+    # (see #9206 for a broader description of the issue).
+    #
+    # Briefly: ruamel.yaml produces a .pth file when installed with pip that
+    # makes the site installed package the preferred one, even though sys.path
+    # is modified to point to another version of ruamel.yaml.
+    if "ruamel.yaml" in sys.modules:
+        del sys.modules["ruamel.yaml"]
+
+    if "ruamel" in sys.modules:
+        del sys.modules["ruamel"]
+
+    # The following code is here to avoid failures when updating
+    # the develop version, due to spurious argparse.pyc files remaining
+    # in the libs/spack/external directory, see:
+    # https://github.com/spack/spack/pull/25376
+    # TODO: Remove in v0.18.0 or later
+    try:
+        import argparse  # noqa: F401
+    except ImportError:
+        argparse_pyc = os.path.join(spack_external_libs, "argparse.pyc")
+        if not os.path.exists(argparse_pyc):
+            raise
+        try:
+            os.remove(argparse_pyc)
+            import argparse  # noqa: F401
+        except Exception:
+            msg = (
+                "The file\n\n\t{0}\n\nis corrupted and cannot be deleted by Spack. "
+                "Either delete it manually or ask some administrator to "
+                "delete it for you."
+            )
+            print(msg.format(argparse_pyc))
+            sys.exit(1)
+
+    import spack.main  # noqa: E402
+
+    sys.exit(spack.main.main(argv))