Add a linting script (#5627)

Add a dev script to cover all the different linting steps.

changelog.d/5627.misc

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+Add `lint.sh` to the scripts-dev folder which will run all linting steps required by CI.

scripts-dev/lint.sh

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+#!/bin/sh
+#
+# Runs linting scripts over the local Synapse checkout
+# isort - sorts import statements
+# flake8 - lints and finds mistakes
+# black - opinionated code formatter
+
+set -e
+
+isort -y -rc synapse tests scripts-dev scripts
+flake8 synapse tests
+python3 -m black synapse tests scripts-dev scripts