Fixes to support Python 2.6 again.

Details:

- Added Python 2.6 again to .travis.yml (it was removed in commit 4486bcb).

- Replaced the use of dictionary comprehensions in `git/cmd.py` around
  line 800 with the code before that change (in commit 25a2ebf).
  Reason: dict comprehensions were introduced only in Python 2.7.

- Changed the import source for `SkipTest` and `skipIf` from `unittest.case`
  to first trying `unittest` and upon ImportError from `unittest2`.
  This was done in `git/util.py` and in several testcases.
  Reason: `SkipTest` and `skipIf` were introduced to unittest only
  in Python 2.7, and `unittest2` is a backport of `unittest` additions
  to Python 2.6.

- In git/test/lib/helper.py, fixed the definition of `assertRaisesRegex`
  to work on py26.

- For Python 2.6, added the `unittest2` dependency to `requirements.txt`
  and changed `.travis.yml` to install `unittest2`. Because git/util.py
  uses SkipTest from unittest/unittest2, the dependency could not be added
  to `test-requirements.txt`.

- Fixed an assertion in `git/test/test_index.py` to also allow
  a Python 2.6 specific exception message.

- In `is_cygwin_git()` in `git/util.py`, replaced `check_output()` with
  `Popen()`. It was added in Python 2.7.

- Enabled Python 2.6 for Windows:

  - Added Python 2.6 for MINGW in .appveyor.yml.

  - When defining `PROC_CREATIONFLAGS` in `git/cmd.py`, made use of certain
    win32 and subprocess flags that were introduced in Python 2.7, dependent
    on whether we run on Python 2.7 or higher.

  - In `AutoInterrupt.__del__()` in `git/cmd.py`, allowed for `os` not having
    `kill()`. `os.kill()` was added for Windows in Python 2.7 (For Linux, it
    existed in Python 2.6 already).

1 files changed, 4 insertions, 2 deletions

diff --git a/git/test/test_repo.py b/git/test/test_repo.py
index 374a26ee..9ad80ee6 100644
--- a/git/test/test_repo.py
+++ b/git/test/test_repo.py
@@ -11,7 +11,10 @@ import os
 import pickle
 import sys
 import tempfile
-from unittest.case import skipIf
+try:
+    from unittest import skipIf, SkipTest
+except ImportError:
+    from unittest2 import skipIf, SkipTest
 
 from git import (
     InvalidGitRepositoryError,
@@ -53,7 +56,6 @@ from git.test.lib import (
 from git.util import HIDE_WINDOWS_KNOWN_ERRORS, cygpath
 from git.test.lib import with_rw_directory
 from git.util import join_path_native, rmtree, rmfile, bin_to_hex
-from unittest import SkipTest
 
 import functools as fnt
 import os.path as osp