Bump python-postmark from 0.5.8 to 0.6.0

Bumps [python-postmark](https://github.com/themartorana/python-postmark) from 0.5.8 to 0.6.0.
- [Release notes](https://github.com/themartorana/python-postmark/releases)
- [Changelog](https://github.com/themartorana/python-postmark/blob/master/CHANGELOG.md)
- [Commits](themartorana/python-postmark@v0.5.8...v0.6.0)

---
updated-dependencies:
- dependency-name: python-postmark
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

requirements/base.txt

@@ -7,7 +7,7 @@ django-flat-theme==1.1.4
 django-guardian==2.4.0
 django-userena-ce==7.0.3
 easy-thumbnails==2.8.3
-python-postmark==0.5.8
+python-postmark==0.6.0
 Markdown==3.4.4
 Pillow==10.0.0
 raven==6.10.0