Gloe

Adding support for Python version 3.9

New features:
- Gloe now raise a custom error when trying to export .dot graph file without pygraphviz installed.

Dependencies:
- Relaxing the constraint of typing_extensions version in dependencies.

Package cleaning:
- Removing unnecessary dependencies.

New features:
- Adding new Filter transformer to collection package.
- Allowing transformers into collection package to be used with any Iterable.

New features:
- Allowing to use ensurers with any callable that returns a transformer.
- Prevent `debug` transformer to stop the code outside a debug mode.

Documentation:
- General readability improvement.
- Fixing typos.

Tests:
- Adding new tests for ensurers

Bug fixes:
- Allowing to call sync transformers from a running event loop.