Bump dnspython from 2.6.1 to 2.8.0 (#581)

Bumps [dnspython](https://github.com/rthalley/dnspython) from 2.6.1 to 2.8.0.
- [Release notes](https://github.com/rthalley/dnspython/releases)
- [Changelog](https://github.com/rthalley/dnspython/blob/main/doc/whatsnew.rst)
- [Commits](rthalley/dnspython@v2.6.1...v2.8.0)

---
updated-dependencies:
- dependency-name: dnspython
  dependency-version: 2.8.0
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

requirements-dev.txt

@@ -2,7 +2,7 @@
 -r aiosmtpd/docs/requirements.txt
 
 check-manifest==0.50
-dnspython==2.6.1
+dnspython==2.8.0
 flake8==7.1.1
 flake8-2020==1.8.1
 flake8-annotations-complexity==0.0.8