Add a bcond for gevent tests

[skip changelog]

python-zmq.spec

@@ -22,9 +22,16 @@ BuildRequires:  pkgconfig(libzmq)
 BuildRequires:  python%{python3_pkgversion}-devel
 BuildRequires:  python%{python3_pkgversion}-pytest
 BuildRequires:  python%{python3_pkgversion}-tornado
-BuildRequires:  python%{python3_pkgversion}-gevent
 BuildRequires:  python%{python3_pkgversion}-numpy
 
+# The gevent tests are automatically skipped when gevent is not installed.
+# When bootstrapping new Python versions, gevent is often not available until very late.
+# This bcond allows to ship zmq without gevent when needed.
+%bcond gevent 1
+%if %{with gevent}
+BuildRequires:  python%{python3_pkgversion}-gevent
+%endif
+
 %description %{common_description}