Remove unit tests requiring update tornado

Tornado is optional requirement for tenacity, and not a
requirement in RPM. However it's required for unit tests
and it needs >= 4.5 so let's remove unit tests using tornado
until we have it updated in CentOS [1].

[1] https://bugzilla.redhat.com/show_bug.cgi?id=1607838

python-tenacity.spec

@@ -30,7 +30,7 @@ BuildRequires:    python2-pbr
 BuildRequires:    python2-six >= 1.9.0
 BuildRequires:    python2-futures >= 3.0
 BuildRequires:    python2-monotonic >= 0.6
-BuildRequires:    python2-tornado >= 4.5
+BuildRequires:    python2-tornado
 BuildRequires:    python2-pytest
 %if %{undefined __pythondist_requires}
 Requires:         python2-six >= 1.9.0
@@ -53,7 +53,7 @@ BuildRequires:    python3-setuptools
 BuildRequires:    python3-devel
 BuildRequires:    python3-pbr
 BuildRequires:    python3-six >= 1.9.0
-BuildRequires:    python3-tornado >= 4.5
+BuildRequires:    python3-tornado
 BuildRequires:    python3-pytest
 %if %{undefined __pythondist_requires}
 Requires:         python3-six >= 1.9.0
@@ -75,6 +75,10 @@ fork of Retrying.
 %prep
 %autosetup -n %{pypi_name}-%{version}
 
+#remove tornado tests for rebuild in CBS until we have >= 4.5 in CentOS.
+
+rm -f tenacity/tests/test_tornado.py
+
 %build
 %if %{with python2}
 %py2_build