gh-37486: sagemath-{objects,categories} `pyproject.toml`: Add 'external' section according to draft PEP 725
    
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As done in #37482 (unbundled from #37446) for **sagemath-standard**.

- This information will be used in the skeleton generator in
pyodide/pyodide#4438

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### ⌛ Dependencies

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URL: #37486
Reported by: Matthias Köppe
Reviewer(s): Kwankyu Lee

Author: Release Manager

pkgs/sagemath-categories/pyproject.toml.m4

@@ -37,3 +37,20 @@ include-package-data = false
 
 [tool.setuptools.dynamic]
 version = {file = ["VERSION.txt"]}
+
+[external]
+# External dependencies in the format proposed by https://peps.python.org/pep-0725
+build-requires = [
+  "virtual:compiler/c",
+  "virtual:compiler/cpp",
+  "pkg:generic/pkg-config",
+]
+
+host-requires = [
+  "pkg:generic/gmp",
+  "pkg:generic/mpc",
+  "pkg:generic/mpfr",
+]
+
+dependencies = [
+]

pkgs/sagemath-objects/pyproject.toml.m4

@@ -46,3 +46,20 @@ version = {file = ["VERSION.txt"]}
     "python_debug.h",
 ]
 "sage.rings" = ["integer_fake.h"]
+
+[external]
+# External dependencies in the format proposed by https://peps.python.org/pep-0725
+build-requires = [
+  "virtual:compiler/c",
+  "virtual:compiler/cpp",
+  "pkg:generic/pkg-config",
+]
+
+host-requires = [
+  "pkg:generic/gmp",
+  "pkg:generic/mpc",
+  "pkg:generic/mpfr",
+]
+
+dependencies = [
+]