gh-36363: Fix dependencies of `packaging`, update `flit_core` to 3.9.0, `pip` to 23.2.1
    
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<!-- Why is this change required? What problem does it solve? -->
In a previous upgrade of `packaging`, its build system was changed to
`flit_core`, but we forgot to update the dependencies. This can lead to
errors as seen in `debian-bullseye-standard-sitepackages` (https://githu
b.com/mkoeppe/sage/actions/runs/6344439483/job/17248093225). We fix it
here.

We also upgrade the packages `flit_core` and `pip`.

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URL: #36363
Reported by: Matthias Köppe
Reviewer(s): Dima Pasechnik

build/pkgs/flit_core/checksums.ini

@@ -1,5 +1,5 @@
 tarball=flit_core-VERSION.tar.gz
-sha1=fc82acf2a93d3bf6ce41ed1a1160f5be51ffda2a
-md5=4a4791e97f3a8c9a1645064a35636ee6
-cksum=2732326458
+sha1=05d6d7a3df5bdbd9df1b37a65662e5dbe94f23fd
+md5=3bc52f1952b9a78361114147da63c35b
+cksum=1703912415
 upstream_url=https://pypi.io/packages/source/f/flit_core/flit_core-VERSION.tar.gz

build/pkgs/flit_core/package-version.txt

@@ -1 +1 @@
-3.7.1
+3.9.0

build/pkgs/packaging/dependencies

@@ -1,4 +1,4 @@
- | setuptools pip wheel pyparsing setuptools_wheel $(PYTHON)
+ | flit_core pip $(PYTHON)
 
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