Python cpp implementation doesn't build with python 2.6

[xfxyjwf]

Failing travis test: https://travis-ci.org/google/protobuf/jobs/77443313

Error message:

google/protobuf/pyext/message.cc:1783:14: error: ‘PyFloat_STR_PRECISION’ was not declared in this scope

This symbol is introduced in a down-integration commit (i.e., first added in our internal code base).

[haberman]

Looking into this.

I'm not sure this needs to hold up the release. Python 2.6 is extremely old and we only decided to support it because RedHat might still be distributing it. If only pure-Python worked for this beta release on Python 2.6, I think that would be acceptable.

[haberman]

I think I can fix this bug relatively easily. Do you want to send a PR?

[xfxyjwf]

We are releasing beta-1 without this. Let's fix it before merging beta-1 to master though.

[QuLogic]

Just as a side note.