Pytest-terra-fixt

- replace utf-8 with cp037 serialization
- create cache hash based on instance attributes that are not functions or classes
- skip adding to cache if tftest method output is None
- Remove private _execute_command tests given fixture-level tests cover its functionality
- run gh workflow jobs in parallel
- add terra binaries to unit testing gh action job

Bug Fixes:
- add {} default for request.param within _execute_comand() so that `terra_setup` fixtures that aren't parameterized raise attribute not found error

The release removes the `terra_cache` and `terra_factory` fixtures introduced in `v0.5`. The caching feature will still be present but in the form of fixtures that wrap around common terra* commands. The following fixtures are:
- `terra_setup`
- `terra_plan`
- `terra_apply`
- `terra_output`

See README.md for more info

Change `terra` fixture from function-scoped to session-scoped

Fixes:
- `terra` and `terra_factory` fixtures now return the tftest object if no command kwarg is passed
- Updates to `terra_cache` dictionary will not instantiate a new TfTestCache class and instead just overwrite class attributes

Removed Fixtures
`tf`
`tf_factory`
`terraform_version`
`terragrunt_version`


Added Fixtures
`terra`: Function-scoped parametrizeable fixture that wraps around `tftest.TerraformTest` or `tftest.TerragrunTest` classes (depending on `binary` kwarg). Features caching of terra* command outputs by specificying the keyword argument `use_cache`.
`terra_factory`: Session-scope factory fixture that wraps around `tftest.TerraformTest` or `tftest.TerragrunTest` classes (depending on `binary` kwarg). This fixture is essentially the `terra` fixture as a factory fixture.
`terra_cache`: Session-scope fixture used as a cache for both `terra` and `terra_factory` fixtures