Add some notes on test dependencies

[skip changelog]

python-zmq.spec

@@ -86,10 +86,17 @@ License:        BSD-3-Clause
 Requires:       python%{python3_pkgversion}-zmq = %{version}-%{release}
 %py_provides    python%{python3_pkgversion}-pyzmq-tests
 
+# Tests in zmq/tests/mypy.py require mypy, but see:
+# https://docs.fedoraproject.org/en-US/packaging-guidelines/Python/#_linters
+#
+# Some tests in zmq/tests/test_context.py and zmq/tests/test_socket.py require
+# pyczmq, which is not packaged and has not been updated in a decade.
+#
 BuildRequires:  python%{python3_pkgversion}-pytest
 Requires:       python%{python3_pkgversion}-pytest
 BuildRequires:  python%{python3_pkgversion}-tornado
 Requires:       python%{python3_pkgversion}-tornado
+# Enable more tests in zmq/tests/test_message.py:
 BuildRequires:  python%{python3_pkgversion}-numpy
 Requires:       python%{python3_pkgversion}-numpy
 %if %{with gevent}