Revert "Run Python tests in 8 threads (#3206)" (#3264)

This reverts commit 56a4466d0a.

Seems that flackiness increased after this commit, while the time
decrease was a couple of seconds.
With every regular Python test spawing 1 etcd, 3 safekeepers, 1
pageserver, few CLI commands and post-run cleanup hooks, it might be
hard to run many such tests in parallel.

We could return to this later, after we consider alternative test
structure and/or CI runner structure.

.github/actions/run-python-test-set/action.yml

@@ -123,8 +123,8 @@ runs:
           exit 1
         fi
         if [[ "${{ inputs.run_in_parallel }}" == "true" ]]; then
-          # -n8 uses eight processes to run tests via pytest-xdist
-          EXTRA_PARAMS="-n8 $EXTRA_PARAMS"
+          # -n4 uses four processes to run tests via pytest-xdist
+          EXTRA_PARAMS="-n4 $EXTRA_PARAMS"
 
           # --dist=loadgroup points tests marked with @pytest.mark.xdist_group
           # to the same worker to make @pytest.mark.order work with xdist