Update pylint requirement from ~=2.3.1 to ~=2.6.0 (#225)

Updates the requirements on [pylint](https://github.com/PyCQA/pylint) to permit the latest version.
- [Release notes](https://github.com/PyCQA/pylint/releases)
- [Changelog](https://github.com/PyCQA/pylint/blob/master/ChangeLog)
- [Commits](pylint-dev/pylint@pylint-2.3.1...pylint-2.6.0)

Signed-off-by: dependabot[bot] <[email protected]>

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

Author: dependabot[bot]

requirements.txt

@@ -5,7 +5,7 @@ Flask-Gzip~=0.2
 flask_sqlalchemy~=2.3.2
 psycopg2-binary~=2.8.6
 Flask-Migrate~=2.2.1
-pylint~=2.3.1
+pylint~=2.6.0
 gunicorn~=19.7.1
 csh_ldap~=2.1.0
 onesignal-sdk~=1.0.0