Restrict pylint/pytest versions and bump CI tags
Description
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.gitlab-ci.yml: Bump tags for testsuite images These new images come with pinned versions of pytestandpylint:- 
pylint:== 2.1.1
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pytest:>= 3.7
 
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dev-requirements.txt: Pin versions of pytest and pylint These packages should only be updated as a deliberate change and the code should be tested with the new versions to ensure that the test still pass. Any future updates should also happen in sync with the buildstream-docker-images repository so that the testsuite base images also enforce the same restrictions for the development requirements. Replaces !684 (closed). 
Changes proposed in this merge request:
- .gitlab-ci.yml: Bump tags for testsuite images
- dev-requirements.txt: Pin versions of pytest and pylint
Edited  by Chandan Singh