CLN: Misc PY2/3 compat functions
- [x] xref #25725
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

- Import `from pandas.compat.chainmap import DeepChainMap` directly where used
- Removed `isidentifier`, `str_to_bytes`, `bytes_to_str`, `bind_method`