New release for pandas 1.6

[andrewgsavage]

#133 contains updates for pandas 1.6. I think it'll work for versions 1.4 and 1.5 too but isn't tested against them.

It's currently testing against a fork of pint, which will need merging into pint. Then will need a new release of pint to specify in the pint-pandas requirements.

It's also using a fork of pandas to test against, making it take longer to run tests. There's a PR to update pandas pandas-dev/pandas#48920 I don't think this is strictly necessary for a pint-pandas release.

To do list

• Merge changes to pint

• New pint release

• Update CI & pint requirement to new release

• New pint-pandas release

• Merge changes to pandas

• New pandas release

• Update CI & pandas requirement to new release