TYP: Fix chainmap typing for mypy 0.740+
This will otherwise raise a failure during a `mypy` update. Also increase type precision of this file to 100%.

- [ ] closes #xxxx
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

cc @simonjayhawkins 
cc @simonjayhawkins  note we are currently pinned to 0.73 for a number of issues. > note we are currently pinned to 0.73 for a number of issues.

Yes, I'm aware of them and this a tiny step to get rid of them. > > note we are currently pinned to 0.73 for a number of issues.
> 
> Yes, I'm aware of them and this a tiny step to get rid of them.

great that's what I figured, thanks! PR looks good to me.

There has been a build-in collections.ChainMap since python 3.3. Is this version different or could we switch over to the builtin version? > PR looks good to me.
> 
> There has been a build-in collections.ChainMap since python 3.3. Is this version different or could we switch over to the builtin version?

The `typing` one used here is simply a typed alias for it that exists since Python 3.6. > note we are currently pinned to 0.73 for a number of issues.

This will help for 0.73 too...

#30539 pandas\compat\chainmap.py:4: error: Implicit generic "Any". Use "typing.ChainMap" and specify generic parameters
 thanks @xhochy 