Valid8

* internal custom copy of `typing_inspect` module was correct for old versions of python but created a bug on new versions of python: optionality detection was not working anymore when using `Optional[]`. Fixes [16](https://github.com/smarie/python-valid8/issues/16)

* removed dependency to `typing_inspect` module so that the module also runs with very old versions of typing.py

* Fixed [14](https://github.com/smarie/python-valid8/issues/14)

* Now relying on typing_inspect to check if an argument is nonable, with support of `TypeVar` and `Union` including nesting.

* Inline validation:

- `validate` has a new argument `custom` where you can provide a callable method, or a list (same than for the decorators)

* Examples:

- one page per example - now with a dedicated test in the sources

* mini-lambda support: now providing an `Instance_of` mini-lambda equivalent of `instance_of`, to raise proper `TypeError`. See example 3 for usage

* Inline validation:

- `validate` has several new arguments: `subclass_of`, `contains`, `subset_of`, `superset_of`, `length`, `equals`
- `validator` has a new argument: `subclass_of`
- New base function `assert_subclass_of`
- `instance_of` and `subclass_of` are now dual functions: they can both serve as a function generators or as a simple functions for inline validation for example inside a `validator` context manager.

* Decorators:

- 2 new function generators `has_length` and `contains`

* Bug fixes:

- Fixed bug [11](https://github.com/smarie/python-valid8/issues/11) where `ValidationError` could not be correctly printed in case of a templating issue.

* New examples in the documentation