Set job-level continue-on-error for Python 3.13 on Windows

It is specifically that combination that breaks some things. The
underlying cause is a change in `isabs` to correctly report that
paths on Windows that start with `\` that is not part of a
construction such as `\\?\` are not absolute paths. (See gitpython-developers#1955 and
links therein.) The change to `isabs` starts in 3.13, and it is
only applicable to Windows, so only that combination is affected.

.github/workflows/pythonpackage.yml

@@ -23,13 +23,16 @@ jobs:
           python-version: "3.7"
           os-ver: "22.04"
         - experimental: false
-        - python-version: "3.13"
+        - os-type: windows
+          python-version: "3.13"
           experimental: true
 
       fail-fast: false
 
     runs-on: ${{ matrix.os-type }}-${{ matrix.os-ver }}
 
+    continue-on-error: ${{ matrix.experimental }}
+
     defaults:
       run:
         shell: bash --noprofile --norc -exo pipefail {0}