README: Three-space indents for nested enumerated lists

Apparently two spaces doesn't cut it.  This change fixes:

  $ rst2html.py --strict README
  README:5: (INFO/1) Enumerated list start value not ordinal-1: "3" (ordinal 3)
  Exiting due to level-1 (INFO) system message.

I also use a bullet list for dependencies, because the order in which
you install them doesn't matter to me.

Signed-off-by: W. Trevor King <wking@tremily.us>
setup.py: Claim support for Python 3.3

We've supported 3.3 for the whole rss2email 3.x branch, but I forgot
to mention it in the trove classifiers.

Signed-off-by: W. Trevor King <wking@tremily.us>
Bump to version 3.7

Signed-off-by: W. Trevor King <wking@tremily.us>