Update .gitlab-ci.yml to run flake8

- should not fail a build, but just as a cautionary kind of warning
- also update to `stage` keyword
5 jobs for 103-flake8 in 2 minutes and 50 seconds
Name Stage Failure
failed
Go-flake8 Lint There is an unknown failure, please try again
go/templatetags/go_extras.py:20:1: E302 expected 2 blank lines, found 1
go/templatetags/go_extras.py:40:1: E302 expected 2 blank lines, found 1
go/templatetags/go_extras.py:45:80: E501 line too long (80 > 79 characters)
go/templatetags/test_go_extras.py:17:1: E302 expected 2 blank lines, found 1
go/templatetags/test_go_extras.py:29:80: E501 line too long (80 > 79 characters)
go/templatetags/test_go_extras.py:58:80: E501 line too long (80 > 79 characters)
go/templatetags/test_go_extras.py:62:80: E501 line too long (80 > 79 characters)
go/templatetags/test_go_extras.py:74:5: E303 too many blank lines (2)
ERROR: Job failed: exit code 1