[1.10.x] Fixed #26645 -- Fixed potential makemessages failures on Windows.

This was reflected by 27 i18n test errors (not failures) on such
platform caused by corrupt intermediate PO catalog files fed to
`msgmerge(1)`.

Made it handle in-memory PO/POT file contents with normalized line
separators (`'\n'`).

We need to perform manually this after using `subprocess.Popen` as we
can't pass it `universal_newlines=True`.

Also fixes #26670 in the process as this commit refactors the same code
section.

Backport of 6ab0d135 from master