re6st: fix build on OS with Python 3

For CentOS_8 & CentOS_8_Stream, I give up:
- nothing provides python, python2, python3 or /usr/bin/python
  (actually, there's a python2 package on CentOS_8_Stream, but only to ship
  a /usr/bin/python2 that tells the user not to use Python 2... wtf ?)
- by default, there exists /usr/libexec/platform-python and it's functional
  but the build of openssl requires any of python{,2,3} in PATH, which means
  that a probable solution is to:
  - conditionally depend on 'python' only `%if 0%{?centos_ver} != 8`
  - change Makefile.in to add a symlink to /usr/libexec/platform-python
    in a temporary directory that we add to $PATH during the build
- ... but it's probably fine to use packages for CentOS_7