Add python and platform details to --version

On Flake8 2.x we added the information about the implementation,
version, and operating system to the --version output to make helping
users easier. In short they can pretty simply just give us the output
from

    flake8 --version

And we can get a lot of the information that we need.

src/flake8/options/manager.py

@@ -236,8 +236,10 @@ def generate_versions(self, format_str='%(name)s: %(version)s'):
 
     def update_version_string(self):
         """Update the flake8 version string."""
-        self.parser.version = (self.version + ' (' +
-                               self.generate_versions() + ')')
+        self.parser.version = (
+            self.version + ' (' + self.generate_versions() + ') ' +
+            utils.get_python_version()
+        )
 
     def generate_epilog(self):
         """Create an epilog with the version and name of each of plugin."""

src/flake8/utils.py

@@ -4,6 +4,7 @@
 import inspect
 import io
 import os
+import platform
 import re
 import sys
 
@@ -290,3 +291,19 @@ def parameters_for(plugin):
         parameters.pop('self', None)
 
     return parameters
+
+
+def get_python_version():
+    """Find and format the python implementation and version.
+
+    :returns:
+        Implementation name, version, and platform as a string.
+    :rtype:
+        str
+    """
+    # The implementation isn't all that important.
+    try:
+        impl = platform.python_implementation() + " "
+    except AttributeError:  # Python 2.5
+        impl = ''
+    return '%s%s on %s' % (impl, platform.python_version(), platform.system())

tests/unit/test_option_manager.py

@@ -4,6 +4,7 @@
 
 import pytest
 
+from flake8 import utils
 from flake8.options import manager
 
 TEST_VERSION = '3.0.0b1'
@@ -162,7 +163,8 @@ def test_update_version_string(optmanager):
 
     assert optmanager.version == TEST_VERSION
     assert (optmanager.parser.version == TEST_VERSION + ' ('
-            'Testing 100: 0.0.0, Testing 101: 0.0.0, Testing 300: 0.0.0)')
+            'Testing 100: 0.0.0, Testing 101: 0.0.0, Testing 300: 0.0.0) ' +
+            utils.get_python_version())
 
 
 def test_generate_epilog(optmanager):