Bump idna from 2.7 to 3.7 in /tests

Bumps [idna](https://github.com/kjd/idna) from 2.7 to 3.7.
- [Release notes](https://github.com/kjd/idna/releases)
- [Changelog](https://github.com/kjd/idna/blob/master/HISTORY.rst)
- [Commits](kjd/idna@v2.7...v3.7)

---
updated-dependencies:
- dependency-name: idna
  dependency-type: direct:production
...

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

tests/requirements_test.txt

@@ -1,6 +1,6 @@
 coverage == 3.7.1 ; python_version == '3.2'
 coverage ; python_version != '3.2'
 coveralls
-idna == 2.7 ; python_version == '2.6'
+idna == 3.7 ; python_version == '2.6'
 pycparser == 2.18 ; python_version == '2.6'
 pyyaml ; python_version != '3.4'