Bump flake8 version to 3.4.0
Bump flake8 version to 3.4.0

mozilla/snippets-service,mozmar/snippets-service,glogiotatidis/snippets-service,glogiotatidis/snippets-service,mozmar/snippets-service,mozilla/snippets-service,glogiotatidis/snippets-service,mozmar/snippets-service,mozmar/snippets-service,mozilla/snippets-service,mozilla/snippets-service,glogiotatidis/snippets-service