sagemathgh-36751: Switch sage-conf to modern editable install

    
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This modernization eliminates some deprecation warnings during
installation. It may help solve the problem of the disappearing sage-
conf (https://groups.google.com/g/sage-
release/c/J6mGYH56FKA/m/m9yHivCWAgAJ, https://groups.google.com/g/sage-
release/c/dvPti2UkyjQ/m/2jeUZROzAwAJ, https://groups.google.com/g/sage-
release/c/DeqhtAgi2es/m/DYq13owvAQAJ).

No change to how sagelib is installed; this will be taken care of
separately (sagemath#34209).

In combination with sagemath#36562, which adds `pyproject.toml` for sage-
docbuild, sage-setup, sage-sws2rst, also these distributions are
switched to the modern editable install.

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### ⌛ Dependencies

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URL: sagemath#36751
Reported by: Matthias Köppe
Reviewer(s): John H. Palmieri

build/bin/sage-dist-helpers

@@ -289,8 +289,6 @@ sdh_pip_install() {
 
 sdh_pip_editable_install() {
     echo "Installing $PKG_NAME (editable mode)"
-    # Until https://github.com/sagemath/sage/issues/34209 switches us to PEP 660 editable wheels
-    export SETUPTOOLS_ENABLE_FEATURES=legacy-editable
     python3 -m pip install --verbose --no-deps --no-index --no-build-isolation --isolated --editable "$@" || \
         sdh_die "Error installing $PKG_NAME"
 }

build/pkgs/sagelib/spkg-install.in

@@ -51,6 +51,8 @@ if [ "$SAGE_EDITABLE" = yes ]; then
     # and renamed the distribution to "sagemath-standard").  There is no clean way to uninstall
     # them, so we just use rm.
     (cd "$SITEPACKAGESDIR" && rm -rf sage sage-[1-9]*.egg-info sage-[1-9]*.dist-info)
+    # Until https://github.com/sagemath/sage/issues/34209 switches us to PEP 660 editable wheels
+    export SETUPTOOLS_ENABLE_FEATURES=legacy-editable
     time sdh_pip_editable_install .
 
     if [ "$SAGE_WHEELS" = yes ]; then