Deprecate/warn usage of yaml.load(input)
The `load` and `load_all` methods will issue a warning when they are
called without the 'Loader=' parameter. The warning will point to a URL
that is always up to date with the latest information on the usage of
`load`.
There are several ways to stop the warning:
* Use `full_load(input)` - sugar for `yaml.load(input, FullLoader)`
* FullLoader is the new safe but complete loader class
* Use `safe_load(input)` - sugar for `yaml.load(input, SafeLoader)`
* Make sure your input YAML consists of the 'safe' subset
* Use `unsafe_load(input)` - sugar for `yaml.load(input, UnsafeLoader)`
* Make sure your input YAML consists of the 'safe' subset
* Use `yaml.load(input, Loader=yaml.<loader>)`
* Or shorter `yaml.load(input, yaml.<loader>)`
* Where '<loader>' can be:
* FullLoader - safe, complete Python YAML loading
* SafeLoader - safe, partial Python YAML loading
* UnsafeLoader - more explicit name for the old, unsafe 'Loader' class
* yaml.warnings({'YAMLLoadWarning': False})
* Use this when you use third party modules that use `yaml.load(input)`
* Only do this if input is trusted
The above `load()` expressions all have `load_all()` counterparts.
You can get the original unsafe behavior with:
* `yaml.unsafe_load(input)`
* `yaml.load(input, Loader=yaml.UnsafeLoader)`
In a future release, `yaml.load(input)` will raise an exception.
The new loader called FullLoader is almost entirely complete as
Loader/UnsafeLoader but it does it avoids all known code execution
paths. It is the preferred YAML loader, and the current default for
`yaml.load(input)` when you get the warning.
Here are some of the exploits that can be triggered with UnsafeLoader
but not with FullLoader:
```
python -c 'import os, yaml; yaml.full_load("!!python/object/new:os.system [echo EXPLOIT!]")'`
python -c 'import yaml; print yaml.full_load("!!python/object/new:abs [-5]")'
python -c 'import yaml; yaml.full_load("!!python/object/new:eval [exit(5)]")' ; echo $?
python -c 'import yaml; yaml.full_load("!!python/object/new:exit [5]")' ; echo $?
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