Add python3 to cc7

Seems like CMSSW_12 (and later releases) need python3. See this example of trying to setup CMSSW_12_0_0 in cmscloud/cc7-cms:latest

$ docker run --rm -it --user root -v /cvmfs:/cvmfs cmscloud/cc7-cms
::: Setting up CMS environment (works only if /cvmfs is mounted on host) ...
::: Setting up CMS environment... [done]
$ cmsrel CMSSW_12_0_0
which: no python3 in 
(/usr/sue/sbin:/usr/sue/bin:/cvmfs/cms.cern.ch/common:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin)
/usr/bin/env: python3: No such file or directory
$ yum install -y python3
.
.
.
Complete!
$ which python3
/usr/bin/python3
$ cmsrel CMSSW_12_0_0
$ ls CMSSW_12_0_0 
biglib  bin  cfipython  config  doc  include  lib  logs  objs  python  src  static  test  tmp