clarify use of pytest-run-parallel marks

Author: ngoldbaum

tests/test_bcrypt.py

@@ -467,6 +467,7 @@ def test_kdf_no_warn_rounds():
     bcrypt.kdf(b"password", b"salt", 10, 10, True)
 
 
+# Python warning state is global
 @pytest.mark.thread_unsafe()
 def test_kdf_warn_rounds():
     with pytest.warns(UserWarning):
@@ -500,7 +501,8 @@ def test_2a_wraparound_bug():
     )
 
 
-@pytest.mark.thread_unsafe()
+# this test spawns threads and is slow, so don't run it in many threads
+@pytest.mark.parallel_threads(1)
 def test_multithreading():
     def get_id():
         return uuid.uuid4().bytes