Fix python tests (#7960)

* Don't generate types unless --python-typing specified

Fixes https://github.com/google/flatbuffers/issues/7944

* Fix incorrect import statements

Fixes https://github.com/google/flatbuffers/issues/7951

* Fix $PYTHONPATH in PythonTest.sh

Regressed from https://github.com/google/flatbuffers/pull/7529

* PythonTest: fail if something goes wrong

GitHub Actions runs `bash PythonTest.sh`, and thus failures were not
visible.

* Build flatc for Python tests

* Regenerate codes

---------

Co-authored-by: Derek Bailey <derekbailey@google.com>

.github/workflows/build.yml

@@ -467,6 +467,9 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v3
+    - name: flatc
+      # FIXME: make test script not rely on flatc
+      run: cmake -G "Unix Makefiles" -DCMAKE_BUILD_TYPE=Release -DFLATBUFFERS_BUILD_TESTS=OFF -DFLATBUFFERS_INSTALL=OFF -DFLATBUFFERS_BUILD_FLATLIB=OFF -DFLATBUFFERS_BUILD_FLATHASH=OFF -DFLATBUFFERS_STRICT_MODE=ON . && make -j
     - name: test
       working-directory: tests
       run: bash PythonTest.sh