overviewer: add warning for CentOS/RHEL 6

If you're using CentOS/RHEL 6, you probably already can't run
Overviewer due to our dependency on argparse breaking python2.6
compatibility. However, if you somehow managed to get python2.7
working on CentOS 6, you now get a nice warning telling you to
finally get off your butt and use something else.

This warning can be dropped once we move to Python 3, which is why
this code was added in the first place: CentOS 6 has no Python 3
package that is still supported by upstream, not even in EPEL.

overviewer.py

@@ -63,6 +63,20 @@ def main():
                             "permissions instead. Overviewer does not need access to "
                             "critical system resources and therefore does not require "
                             "root access.")
+        try:
+            with open("/etc/redhat-release", "r") as release_f:
+                rel_contents = release_f.read()
+                try:
+                    major_rel = re.search(r'\d(\.\d+)?', rel_contents).group(0).split('.')[0]
+                    if major_rel == "6":
+                        logging.warning(
+                            "We will be dropping support for this release of your distribution "
+                            "soon. Please upgrade as soon as possible, or you will not receive "
+                            "future Overviewer updates.")
+                except AttributeError:
+                    pass
+        except IOError:
+            pass
 
     try:
         cpus = multiprocessing.cpu_count()