Updated code to Python 3 compatibility.

Did a "futurize -1 -w .", made additional fixes, and committed. Then did a "futurize -w .", which pointed out that I needed to update a example
program in examples/ that used the old pex.logging code. I updated that to use pex.log.

Made an additional fix daf_base. Previously doing this:

from lsst.daf.base import PropertySet
root = PropertySet()
root.set("blank", "")

worked. Under Python 3 this gave an error. Added an additional check in daf_base/tickets/DM-7169 for this. I'm unsure if there's a "more correct" way of checking for this condition.

These changes have been committed and pushed to their respective ticket branches.

Can you please take a look at these changes? I believe they are complete.

There are changes in

ctrl_events/tickets/DM-7169
daf_base/tickets/DM-7169

The fix for daf_base was because setting a PropertySet value to "" broke baseLib.i
(see comment above this one)

And I should say, while this change makes things backwards compatible, there's still some question in my mind about whether or not empty strings should be allowed to be set in a PropertySet in this first place. I do not believe empty sets or empty tuples are allowed.

I think the fix is still correct for strings in general. It's orthogonal to the question of whether an empty string is acceptable.

Oh, I see. For other strings that code doesn't matter because it's only using the exemplar to work out the type.

Minor comments on PR. Not sure if we want to have pex_logging and log in the same package. Looks straightforward. Merge if it builds fine on Jenkins for py2 and py3 (py3 as far as ctrl_events, py2 for lsst_distrib).

It occurred to me on the way to lunch that I might have another pex_logging creeping around in there somewhere still, after that find in examples/.

I'll get rid of all those references.

I took out the last references to pex.logging, and sent it through the Jenkins build, which completed successfully for py2 and py3.

changes merged to daf_base
$ git push
Counting objects: 1, done.
Writing objects: 100% (1/1), 245 bytes | 0 bytes/s, done.
Total 1 (delta 0), reused 0 (delta 0)
To git@github.com:lsst/daf_base.git
a28b38a..3a3e718 master -> master

changes merged to ctrl_events
cd ctrl_events
bash-4.2$ git push
Counting objects: 1, done.
Writing objects: 100% (1/1), 245 bytes | 0 bytes/s, done.
Total 1 (delta 0), reused 0 (delta 0)
To git@github.com:lsst/ctrl_events.git
b6b4f2a..8e83922 master -> master