sagemathgh-38875: libsemigroup upgrade to 2.7.3
    
necessary for GAP 4.13.1

This was forgotten in sagemath#38804


### 📝 Checklist

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- [ x] The description explains in detail what this PR is about.
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preview.

### ⌛ Dependencies

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URL: sagemath#38875
Reported by: Dima Pasechnik
Reviewer(s): Marc Culler

Author: Release Manager

build/pkgs/libsemigroups/checksums.ini

@@ -1,4 +1,4 @@
 tarball=libsemigroups-VERSION.tar.gz
-sha1=86375824b47ce4b0e23570122e873f67136d0c0a
-sha256=6214fd9e87af3834ff5eb6377cde1cbef76c74b233e1b0c4d15af1d2311692b4
+sha1=9c8e73b18a4964135b63e03b2f36b874742edd62
+sha256=d4d88a11651c7d7d497f847fea97e3ff60a39b25b851c1d0d7ccf41e052612be
 upstream_url=https://github.com/libsemigroups/libsemigroups/releases/download/vVERSION/libsemigroups-VERSION.tar.gz

build/pkgs/libsemigroups/package-version.txt

@@ -1 +1 @@
-2.3.2
+2.7.3