Prepare 4.4.0 release metadata

Two small gaps found while checking the build ahead of the release:

- The 4.4.0 changelog heading had no date, unlike every previous entry.
- `long_description_content_type` was never declared, so `twine check`
  warned and PyPI had to fall back to guessing `text/x-rst`. The long
  description is reStructuredText, so the guess was right, but stating
  it removes the warning and stops a future default change from
  silently breaking the project page rendering.

With both applied, `twine check` passes with no warnings.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: PetrDlouhy

README.md

@@ -409,7 +409,7 @@ You may need to do this if you use a custom user model and upgrade Django.
 
 ## Change Log
 
-### 4.4.0
+### 4.4.0 (2026-07-28)
 
 * Dropped support for Django < 3.2 and Python < 3.8
 * Now tested against Django 3.2, 4.2, 5.2, 6.0 and main, on Python 3.8-3.13

setup.py

@@ -69,6 +69,7 @@
     description="a referrals app for Django",
     name="pinax-referrals",
     long_description=LONG_DESCRIPTION,
+    long_description_content_type="text/x-rst",
     version=VERSION,
     url="http://github.com/pinax/pinax-referrals/",
     license="MIT",