gh-39031: Remove pipfile
    
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A few years ago I made some experiements with pipenv and added these
`pipfile`'s in the progress. I no longer think this is a valuable
strategy for sage (e.g `uv` seems to better align with the current
philosohpy). Thus, the pipfiles can be deleted again.


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- [ ] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation and checked the documentation
preview.

### ⌛ Dependencies

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URL: #39031
Reported by: Tobias Diez
Reviewer(s): Frédéric Chapoton

Author: Release Manager

.ci/write-dockerfile.sh

@@ -275,11 +275,11 @@ cat <<EOF
 FROM with-system-packages AS bootstrapped
 #:bootstrapping:
 RUN rm -rf /new /sage/.git
-$ADD Makefile VERSION.txt COPYING.txt condarc.yml README.md bootstrap bootstrap-conda configure.ac sage .homebrew-build-env tox.ini Pipfile.m4 .gitignore /new/
+$ADD Makefile VERSION.txt COPYING.txt condarc.yml README.md bootstrap bootstrap-conda configure.ac sage .homebrew-build-env tox.ini .gitignore /new/
 $ADD config/config.rpath /new/config/config.rpath
 $ADD src/doc/bootstrap /new/src/doc/bootstrap
 $ADD src/bin /new/src/bin
-$ADD src/Pipfile.m4 src/pyproject.toml src/requirements.txt.m4 src/setup.cfg.m4 src/VERSION.txt /new/src/
+$ADD src/pyproject.toml src/requirements.txt.m4 src/setup.cfg.m4 src/VERSION.txt /new/src/
 $ADD m4 /new/m4
 $ADD pkgs /new/pkgs
 $ADD build /new/build

.gitignore

@@ -56,10 +56,6 @@
 
 /src/setup.cfg
 /src/requirements.txt
-/src/Pipfile
-/src/Pipfile.lock
-/Pipfile
-/Pipfile.lock
 
 # Various editors
 *~

Makefile

@@ -175,7 +175,6 @@ bootstrap-clean:
 	rm -f src/doc/en/installation/*.txt
 	find src/doc/en/reference/spkg -name index.rst -prune -o -maxdepth 1 -name "*.rst" -exec rm -f {} \+
 	for a in environment environment-optional src/environment src/environment-dev src/environment-optional; do rm -f $$a.yml $$a-3.[89].yml $$a-3.1[0-9].yml; done
-	rm -f src/Pipfile
 	rm -f src/requirements.txt
 	rm -f src/setup.cfg
 	rm -f build/pkgs/cypari/version_requirements.txt

Pipfile.m4

@@ -227,7 +227,6 @@ save () {
 	src/doc/en/installation/*.txt \
 	$(find src/doc/en/reference/spkg -name index.rst -prune -o -maxdepth 1 -name "*.rst" -print) \
 	environment-3.[89].yml environment-3.1[0-9].yml \
-	src/Pipfile \
 	src/pyproject.toml \
 	src/requirements.txt \
 	src/setup.cfg \