Update idna requirement from <2.9,>=2.5 to >=2.5,<2.10

Updates the requirements on [idna](https://github.com/kjd/idna) to permit the latest version.
- [Release notes](https://github.com/kjd/idna/releases)
- [Changelog](https://github.com/kjd/idna/blob/master/HISTORY.rst)
- [Commits](kjd/idna@v2.5...v2.9)

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

requirements.txt

@@ -1,5 +1,5 @@
 django>=1.11
-idna>=2.5,<2.9
+idna>=2.5,<2.10
 requests>=2.13.0
 Markdown==2.6.11
 MarkupSafe==1.1.1