requirements: Add dependency on ruamel.yaml.clibchandan/dependon-yaml.clib

On Python3.8, when `ruamel.yaml.clib` is not installed, BuildStream ends
up throwing a stack trace on any and all operations. This manifests
itself in the following form:

        $ bst show hello.bst

        Error loading user configuration:
        /venv/lib/python3.8/site-packages/buildstream/data/userconfig.yaml:
        Severely malformed YAML:

        'NoneType' object is not callable

This is also the root cause of downstream issues like
https://gitlab.com/BuildStream/bst-plugins-container/issues/19.

The fix is simple - to add a hard dependency on `ruamel.yaml.clib`, since
we explicitly import `yaml.Cparser` -
https://gitlab.com/BuildStream/buildstream/blob/master/src/buildstream/_yaml.pyx#L298
- that doesn't come from the main `ruamel.yaml` package.

This works fine on Python < 3.8 because `ruamel.yaml` itself has a hard
dependency on `ruamel.yaml.clib` for those versions. The Python 3.8 CI
pipeline also works because our requirements were frozen using a Python
3.7 interpreter and hence already have this requirement.

1 files changed, 1 insertions, 0 deletions

diff --git a/requirements/requirements.in b/requirements/requirements.in
index ce721da33..50bb523da 100644
--- a/requirements/requirements.in
+++ b/requirements/requirements.in
@@ -5,6 +5,7 @@ pluginbase
 protobuf >= 3.6
 psutil
 ruamel.yaml >= 0.16
+ruamel.yaml.clib >= 0.1.2
 setuptools
 pyroaring
 ujson