Rewrite unit formatter for pint 0.24 and earlier

[aulemahal]

Fixes #522 and hgrecco/pint#2024.

• Rewrite of the custom unit formatter to properly use the tools of pint instead of relying on regex.
• To preserve backwards compatibility the pint.formatter call changes according to the installed version of pint.
• Before, f"{u:cf}" and f"{u:~cf}" returned the same thing except for "dimensionless". This shortening is done explicitly in the new formatter with a if-else in case it was already done by pint.
• Removed a few unneeded lines
  • "percent" was always printed as "%" for all supported pint versions, so I don't know why it as there.
  • The formatter's declaration never raised ImportError, so I think the try-except was a legacy from something else.
• Added some warnings to the documentation about non-udunits units.
• Added dimensionless to the tests.

I tested this with pint 0.24.1 and 0.23. I also did a very minimal test with pint 0.22 down to 0.12. This seems to support pint >= 0.18, like the previous version did.

With pint < 0.24 the only difference this makes is that dimensionless units are returned as "" with {u:cf} and {u:~cf}, where as the previous version returned "dimensionless" for {u:cf}.

[dcherian]

Failure is real.

[keewis]

I've been a bit away from pint, so I'll have to first look into how it changed in the most recent version before giving any thorough reviews, but at a quick glance this looks fine.

[dcherian]

but at a quick glance this looks fine.

Thanks! no need for a thorough review.