Fix semi-circular import issue in python3.6

[Chiiizzzy]

In towhee/engine/operator_io, import reader and writer in init.py, should use

from towhee.engine.operator_io import reader as io_reader
from towhee.engine.operator_io import writer as io_writer

instead of

import towhee.engine.operator_io.reader as io_reader
import towhee.engine.operator_io.writer as io_writer

which cause error in python 3.6

[codecov-commenter]

Codecov Report

Merging #311 (e11c896) into main (fbfd4dd) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #311   +/-   ##
=======================================
  Coverage   70.46%   70.46%           
=======================================
  Files         209      209           
  Lines        7573     7573           
  Branches     1004     1004           
=======================================
  Hits         5336     5336           
  Misses       1984     1984           
  Partials      253      253           

Impacted Files	Coverage Δ
towhee/engine/operator_io/__init__.py	78.94% <100.00%> (ø)

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[fzliu]

/approve
/lgtm

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