Issue #530 - Use SafeLoader for YAML load calls

[nttoole]

The primary changes address using safer version of YAML loads:

1. swapping yaml.load(source,Loader=yaml.Loader) to yaml.safe_load(source). (similar for load_all)
2. swap Yaml object constructors from yaml.add_constructor() to yaml.SafeLoader.add_constructor()
3. update pyyaml version to '6.0.1'

Then we hit a series of pre-commit errors due to newly-corrected configs which now run reorder-python-imports and black on the source code. So to satisfy those, more non-yaml updates were made. For example, AitConfigMissing was renamed AitConfigMissingError.
However, after a while, it became clear that black and flake8 have some contradictory rules ( e.g. 'binary logical operators across multiple lines' vs 'line-too-long' errors). In the end, we disabled checks for the binary-ops-spanning-multi-line checks in flake8, cuz that just seems a cleaner presentation (to me at least).
So there are a lot of line changes related to the above, had I known I would have just avoided satisfying any pre-commit issues and forced the Yaml-related changes only.

flake8 would still complain about source files which weren't even touched for the PR, so in the end the pre-commit was disabled so this PR could be made without touching nearly every source file (a separate PR for that busy-work).

Testing included automated tests, along with running the server with the AIT-GUI plugin and ait_example which successfully showed telemetry graphed in the browser. (Config file was reset to clear out plugins.)

Fixes #530

[MJJoyce]

Looks good to me. +1 to merge as is and resolve any linting / build issues separately.