check-code: new rule to forbid imports of a.b on the same line as other imports

This style of import can trip up 2to3 and cause it to produce invalid
files if one of the imports is supposed to be a relative import. This
prevents that behavior, and in the process exposed a lot of silly
import errors related to the email module.

contrib/check-code.py

@@ -162,6 +162,9 b' pypats = ['
      "tuple parameter unpacking not available in Python 3+"),
     (r'lambda\s*\(.*,.*\)',
      "tuple parameter unpacking not available in Python 3+"),
+    (r'import (.+,[^.]+\.[^.]+|[^.]+\.[^.]+,)',
+     '2to3 can\'t always rewrite "import qux, foo.bar", '
+     'use "import foo.bar" on its own line instead.'),
     (r'(?<!def)\s+(cmp)\(', "cmp is not available in Python 3+"),
     (r'\breduce\s*\(.*', "reduce is not available in Python 3+"),
     (r'\.has_key\b', "dict.has_key is not available in Python 3+"),