Order on msgids included in sphinx.pot is not deterministic

[cocoatomo]

Subject: Order on msgids in sphinx.pot has difference even if its content has no essential changes

Problem

• When .pot files are managed with VCS (such as Git), difference without essential changes bother project manager.

Procedure to reproduce the problem

git clone https://github.com/python/cpython
cd cpython/Doc
make build ALLSPHINXOPTS="-E -b gettext -D gettext_compact=0 -d build/.doctrees . locales/pot"

Error logs / results

• python/python-docs-ja@e23b4d3#diff-45573348f1c20fa58a6bb5ae4fbeb851

Expected results

Treated as "No difference".
More concretely, msgids in sphinx.pot have deterministic order.

Reproducible project / your project

• https://github.com/python/cpython (.pot files are genereted from this project)

Environment info

• OS: Linux/Travis CI
• Python version: 3.6.3
• Sphinx version: 1.7.0