Always pass a Loader argument to yaml.load

[sbesson]

Since PyYAML 5.1, calling yaml.load() without specifying a Loader argument is deprecated. With the release of PyYAML 6.0 yesterday, the Loader is now a mandatory positional argument.

This causes scc commands loading YAML configurations e.g. merge --repository-config (see https://merge-ci.openmicroscopy.org/jenkins/job/BIOFORMATS-push/1006/console ) or the newly introduced bump-conda-version to fail on newly created environments - see also yaml/pyyaml#576

This PR adjusts all usages of yaml.load() to pass Loader=yaml.FullLoader which is equivalent to the former call. This should make the library compatible with both PyYAML 5.x and PyYAML 6.x. The alternative would be to cap PyYAML to 5.x but this feels like a minimal change to maintain compatibility between both lines. The only open question is around the loader to use. feb2c22 uses the FullLoader which is identical to the previous call but there might be a case for tightening it to SafeLoader.

To test this PR:

• create a new Python virtual environment and run /path/to/venv/bin/pip install scc - this should install PyYAML 6.0
• using https://github.com/ome/bio-formats-build, run the following command

  /path/to/venv/bin/scc merge --info master --repository-config=scripts/repositories.yml 
  

  this should fail with the same error as indicated above
• checkout this branch and re-install scc in the same virtual environment /path/to/venv/bin/pip install . Running the same command as above should now execute the command as expected

[sbesson]

Merging and releasing to assess the impact on the rest of the CI pipeline.

[sbesson]

1a54999 pushed (and released) to fix a regression in the bump-version-conda. I had overlooked the fact that ruamel.yaml introduced in #273 is a complete alternative topyyaml. There is likely a case for unifying.