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

xref https://github.com/pandas-dev/pandas/pull/37024#discussion_r503488032

cc @simonjayhawkins 
@jbrockmendel ok ? lgtm. can you merge master and ping on green. CI green thanks @arw2019 