Pyramid-formencode-classic

BACKWARDS INCOMPATIBLE CHANGE
* `form_validate` now requires `return_stash=True` - which is enabled by default.
To support `return_stash=False`, please call `form_validate_simple` instead.
This change was required to streamline typing and better standardize the library.
Background:
`form_validate` was renamed to `_form_validate_core`
the new `form_validate` ensures `return_stash==True` and invokes `_form_validate_core`
the new `form_validate_simple` ensures `return_stash==False` and invokes `_form_validate_core`

* drop py2
* mypy support
* reorganized a lot of code/functions into new files.
the relevant imports should be unchanged.

* packaging fixes

* upgraded black; 20.8b1
* integrated with pre-commit
* upgraded to formencode 2.0.0, which is Python3 compatible!
* debugtoolbar support!
* testing via github actions

integrated pyramid's `add_finished_callback` to cleanup fieldstorage objects.
This can be toggled off by setting `pyramid_formencode_classic.AUTOMATIC_CLEANUP = False` before including the file into pyramid

backwards compatible? NO.
* `FieldInvalid` renamed to `FormFieldInvalid`
* `FormFieldInvalid` inherits from `FormInvalid`
* `CsrfInvalid` inherits from `FormFieldInvalid`
* `raise_FormInvalid` kwarg removed from several places
* `raise_FieldInvalid` kwarg removed from several places
* `error_main` kwarg renamed to `error_main_text`
* `set_error` submitting no message will now raise a ValueError exception; use `clear_error` to remove errors

* streamline api
* new : `FormStash.register_error_main_exception`
* new : `FormStash.fatal_form()`
* new : `FormStash.fatal_field()`
* new : explicit versioning policy
* new : tracking `is_submitted_vars` on form
* `formatters.*` no longer imported into main namespace

added `pyproject.toml`