sagemathgh-37482: src/pyproject.toml: Add 'external' section per draft PEP 725 (unbundled from sagemath#37446)
    
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This is aspirational decoration for future use by skeleton generators by
distributions such as conda, sage-the-distribution, pyodide.

Split out from the disputed sagemath#37446, where it is bundled with a number of
other changes, including: creating a `pyproject.toml` file in
`SAGE_ROOT`, hardcoding versions of Python packages instead of using the
existing `sage_bootstrap` infrastructure, etc. @roed314 @vbraun

**The scope of PRs should be chosen to minimize friction, not to
maximize friction.**
sagemath#36726 (comment)

Author: @mkoeppe, based on @tobiasdiez's sagemath#37446.

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### ⌛ Dependencies

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URL: sagemath#37482
Reported by: Matthias Köppe
Reviewer(s):

Author: Release Manager

src/pyproject.toml

@@ -21,3 +21,68 @@ build-backend = "setuptools.build_meta"
 platforms = [
     'osx-64', 'linux-64', 'linux-aarch64', 'osx-arm64'
 ]
+
+[external]
+# External dependencies in the format proposed by https://peps.python.org/pep-0725
+# In the future, sage-the-distribution can read this information
+build-requires = [
+  "virtual:compiler/c",
+  "virtual:compiler/cpp",
+  "pkg:generic/pkg-config"
+]
+
+host-requires = [
+  "virtual:interface/blas",
+  "pkg:generic/boost",
+  "pkg:generic/brial",
+  "pkg:generic/cddlib",
+  "pkg:generic/cliquer",
+  "pkg:generic/ecl",
+  "pkg:generic/eclib",
+  "pkg:generic/ecm",
+  "pkg:generic/fflas-ffpack",
+  "pkg:generic/fplll",
+  "pkg:generic/flint",
+  "pkg:generic/libgd",
+  "pkg:generic/gap",
+  "pkg:generic/gfan",
+  "pkg:generic/giac",
+  "pkg:generic/givaro",
+  "pkg:generic/glpk",
+  "pkg:generic/gmp",
+  "pkg:generic/gsl",
+  "pkg:generic/iml",
+  "pkg:generic/lcalc",
+  "pkg:generic/libbraiding",
+  "pkg:generic/libhomfly",
+  "pkg:generic/linbox",
+  "pkg:generic/lrcalc",
+  "pkg:generic/m4ri",
+  "pkg:generic/m4rie",
+  "pkg:generic/maxima",
+  "pkg:generic/mpc",
+  "pkg:generic/mpfi",
+  "pkg:generic/mpfr",
+  "pkg:generic/nauty",
+  "pkg:generic/ntl",
+  "pkg:generic/palp",
+  "pkg:generic/pari",
+  "pkg:generic/pari-elldata",
+  "pkg:generic/pari-galdata",
+  "pkg:generic/pari-seadata",
+  "pkg:generic/planarity",
+  "pkg:generic/ppl",
+  "pkg:generic/primesieve",
+  "pkg:generic/primecount",
+  "pkg:generic/qhull",
+  "pkg:generic/rw",
+  "pkg:generic/singular",
+  "pkg:generic/symmetrica",
+  "pkg:generic/sympow",
+]
+
+dependencies = [
+  "pkg:generic/tachyon",
+  "pkg:generic/sagemath-polytopes-db",
+  "pkg:generic/sagemath-elliptic-curves",
+]