Update ci.yaml

[slott56]

Add py313 environment. Make 3.13 the default version.

[slott56]

Python 3.13 support still inadequate.

      building '_re2' extension
      creating build/temp.linux-x86_64-cpython-313
      g++ -fno-strict-overflow -Wsign-compare -DNDEBUG -g -O3 -Wall -fPIC -I/home/runner/work/cel-python/cel-python/.tox/py313/include -I/opt/hostedtoolcache/Python/3.13.5/x64/include/python3.13 -c _re2.cc -o build/temp.linux-x86_64-cpython-313/_re2.o -fvisibility=hidden
      _re2.cc:15:10: fatal error: absl/strings/string_view.h: No such file or directory
         15 | #include "absl/strings/string_view.h"
            |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
      compilation terminated.
      error: command '/usr/bin/g++' failed with exit code 1
      [end of output]