Merge pull request #204 from apple/fix-centos-7

[CentOS 7] Add python3 devel package

Author: shahmishal

swift-ci/master/centos/7/Dockerfile

@@ -26,6 +26,7 @@ RUN yum install --enablerepo=centosplus  -y \
     openssl-devel       \
     pexpect             \
     python-devel        \
+    python3-devel       \
     python-pygments     \
     python-six          \
     python36-pexpect    \