fix: lower bound typing-extensions >= 4.6

[ymerkli]

Changes

Fixes #472

typeguard uses typing_extensions.Buffer, which was only added to typing_extenions>=4.6.0, see here.

This PR adds typing_extenions>=4.6.0 as a requirement.

Checklist

• If this is a trivial change, like a typo fix or a code reformatting, then you can ignore
  these instructions.
• Updating the changelog

[coveralls]

coverage: 94.139%. remained the same
when pulling 76af066 on ymerkli:ymerkli/lower-bound-typing-extensions
into 016f813 on agronholm:master.

[agronholm]

Typeguard already has a higher typing_extensions requirement here. Besides, your PR adds it to build requirements, why?