Require Werkzeug and import safe_utils from there, not Flask

[bryce-carson]

Flask removed the already deprecated function safe_utils in 2.1.0.

This shoudl be enough to resolve #359

[zodman]

works for me!

[joeyespo]

Thanks for the PR, @bryce-carson!

I started looking at this after the 4.6.0 was released, but wanted to spend more time making sure it was still compatible with older Flask/Werkzeug versions. Crediting you in CHANGES.md since you got this PR up so quickly after Flask's recent release 😃

If you're curious, here's what was done for backwards compatibility

• Port Flask's behavior of raising an exception in the error case since Werkzeug's implementation returns None instead (details)
• Fix a test that identified an edge case between Flask/Werkzeug versions (details).

This should allow Grip to continue to function when using older versions Flask and Werkzeug (until support is officially dropped in the next major release of Grip).