install flake8 in python (#413)

Author: justice-adams-apple

swift-ci/dependencies/requirements.txt

@@ -4,4 +4,8 @@ psutil>=5.9.4
 # Pexpect tests are known not to work on Windows, so are disabled.
 # See llvm.org/pr22274.
 pexpect>=4.9.0; sys_platform != 'win32'
-packaging
+packaging
+
+# Needed for swift tests
+flake8
+flake8-import-order