doctor: use -V for python version

`python --version` doesn't work in some (very) old versions of
python. While I doubt this is going to come up very often, it's
theoretically possible someone has an old python first in their
path. (python -V works on all versions I'm aware of, including
python3, and comes before --version in the --help anyway.)

Also skips the warning in case the regexp matches nothing, which
probably shouldn't happen. But if it does we shouldn't produce the
wrong warning.

Library/Homebrew/cmd/doctor.rb

@@ -890,8 +890,10 @@ end
 
 def check_for_bad_python_symlink
   return unless which "python"
-  # Indeed Python --version outputs to stderr (WTF?)
-  `python --version 2>&1` =~ /Python (\d+)\./
+  # Indeed Python -V outputs to stderr (WTF?)
+  `python -V 2>&1` =~ /Python (\d+)\./
+  # This won't be the right warning if we matched nothing at all
+  return if $1.nil?
   unless $1 == "2" then <<-EOS.undent
     python is symlinked to python#$1
     This will confuse build scripts and in general lead to subtle breakage.