CI: fix lint issue
https://github.com/pandas-dev/pandas/runs/577244968

Linting .py code
##[error]./pandas/core/internals/blocks.py:11:1:F401:'pandas._libs.tslibs.Timedelta' imported but unused
Linting .py code DONE
LGTM oh is master broken
merge when ready Thanks @simonjayhawkins 