rpm: install python%{python3_pkgversion}-devel on el7

as RHEL 7.7 support a native python3.6, we cannot hardwire to python3.4
on RHEL7/CentOS7 anymore.

Signed-off-by: Kefu Chai <kchai@redhat.com>
(cherry picked from commit d1e8308)

ceph.spec.in

@@ -222,16 +222,9 @@ BuildRequires:	python%{_python_buildid}-prettytable
 BuildRequires:	python%{_python_buildid}-sphinx
 BuildRequires:	lz4-devel >= 1.7
 %endif
-# python34-... for RHEL, python3-... for all other supported distros
-%if 0%{?rhel}
-BuildRequires:	python34-devel
-BuildRequires:	python34-setuptools
-BuildRequires:	python34-Cython
-%else
-BuildRequires:	python3-devel
-BuildRequires:	python3-setuptools
-BuildRequires:	python3-Cython
-%endif
+BuildRequires:	python%{python3_pkgversion}-devel
+BuildRequires:	python%{python3_pkgversion}-setuptools
+BuildRequires:	python%{python3_pkgversion}-Cython
 # distro-conditional make check dependencies
 %if 0%{with make_check}
 %if 0%{?fedora} || 0%{?rhel}