Remove dependency on pydantic

[abravalheri]

• Closes Dependency on pydantic requires Rust, which is not available on Cygwin. Pre-compiled wheels might also be not available on non-{windows,linux,mac} platforms #195.

In this PR, I propose removing the dependency on pydantic to avoid the installation problems caused by the lack of pre-compiled wheels and/or the Rust toolchain in some platforms (e.g. Cygwin or FreeBSD).

The objective is to make improve the viability of inflect (and other packages that depend on it) as a OS agnostic library.

Main changes in implementation

• Replace the pydantic.validate_call decorator with explicit calls to _validate_type, which is based on isinstance.

Main caveats

• For number_to_words, I just introduced validation for the num argument, not for the other ones. Please let me know if it would be relevant to add validations for the other arguments too.
• Calls previously raising pydantic...ValidationError will not raise TypeError(f"{obj!r} given, but expecting {types}").

[abravalheri]

Close in favour of #199