- Use os.makedirs (I had forgotten about it!)
- Let TempdirManager.write_file call os.path.join for us
- Remove custom command added by test_dist
- Use a skip instead of hiding a method with an underscore
- Address pyflakes warnings

I need this for some tests, and it makes code clearer.  This commit also
changes some assertEqual calls to use (actual, expected) order and fix
some pyflakes warnings.

We now have one function to prepare multipart POST requests, and we use
CRLF, as recommended by the HTTP spec (#10150).  Initial patch by John
Edmonds.