Breaking python2.4 compatibility for bzr-2.4

[John Arbash Meinel]

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This has been discussed a bit in the past, and Martin and I have chatted
about it a bit recently on IRC. I've personally reached my threshold,
and would like to break compatibility with python 2.4 for our next
release (bzr 2.4).

The big motivations for this are:
1) RHEL was the only platform that didn't have python >2.4. It now has
   RHEL 6 which has python 2.6. And while RHEL5 is going to be
   supported for something like 100 more years, RHEL5 was originally
   released in 2007 which probably predates bzr-1.0 (2007-12-14).

   If someone is running anything like an up-to-date bzr on RHEL5 they
   are running custom packages. And there are tons of workarounds for
   them if they must keep RHEL5 and a newer bzr (run the stable 2.3
   series, install a python version that isn't 5 years old, etc.)

2) try/finally in generators. This helps a lot to make cleanup in a
   generator much clearer.

3) try/except/finally, also nice to have. Not critical, but definitely
   nice to have.

4) with: statements. Along the same lines of "make your cleanup
   clearer".

5) If we go all the way to python2.6 compatibility we can start looking
   at doing things to support Python 3.
   (This is my recommendation, since RHEL6 has python2.6, and I don't
   think anything else is strictly blocked for python2.5)

6) PQM is now running the test suite directly with python2.6. We plan
   on having Babune run python2.4 compatibility tests, but that is
   going to be a longer turn-around time from finding out you did
   something not-2.4 (5) compatible and fixing it.

The main argument that I've seen for keeping 2.4 is that if we introduce
a new repository format, then we force people that want to use that
format on their clients to upgrade their server (which could be running
an old crufty RHEL5 with maybe some new bzr packages to get 2a format
compatibility.)

I say, lets cross that bridge when we get there. We can look at
providing the repo format as a backport to a stable series, or as a
plugin for older series. Or we can honestly say "If you want to run new
software, install a newer OS", since there *is* a RHEL which is newer
and supported (non beta) and has the newer python available.

I do have a patch right now which breaks 2.4 compatibility (because of
(2)), but I've certainly wanted some of the other stuff for a while.

I'm willing to have this discussion again before landing my patch, though.

John
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