git-p4: make python2.7 the oldest supported version

Python2.6 and earlier have been end-of-life'd for many years now, and
we actually already use 2.7-only features in the code. Make the version
check reflect current realities.

This also removes the need to explicitly define CalledProcessError if
it's not available.

Signed-off-by: Yang Zhao <yang.zhao@skyboxlabs.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-p4.py

@@ -8,9 +8,8 @@
 # License: MIT <http://www.opensource.org/licenses/mit-license.php>
 #
 import sys
-if sys.hexversion < 0x02040000:
-    # The limiter is the subprocess module
-    sys.stderr.write("git-p4: requires Python 2.4 or later.\n")
+if sys.version_info.major < 3 and sys.version_info.minor < 7:
+    sys.stderr.write("git-p4: requires Python 2.7 or later.\n")
     sys.exit(1)
 import os
 import optparse
@@ -43,21 +42,6 @@ else:
     bytes = str
     basestring = basestring
 
-try:
-    from subprocess import CalledProcessError
-except ImportError:
-    # from python2.7:subprocess.py
-    # Exception classes used by this module.
-    class CalledProcessError(Exception):
-        """This exception is raised when a process run by check_call() returns
-        a non-zero exit status.  The exit status will be stored in the
-        returncode attribute."""
-        def __init__(self, returncode, cmd):
-            self.returncode = returncode
-            self.cmd = cmd
-        def __str__(self):
-            return "Command '%s' returned non-zero exit status %d" % (self.cmd, self.returncode)
-
 verbose = False
 
 # Only labels/tags matching this will be imported/exported