Fix lint-python failure due to Sphinx 3.1.0.

[ueshin]

Sphinx 3.1.0 which was published recently causes the following lint-python failure.

The upper bound was set at #1569. We should fix the failures and remove the upper bound.

/home/runner/miniconda/envs/test-environment/lib/python3.8/site-packages/numpydoc/docscrape.py:377: UserWarning: Unknown section Raises: in the docstring of None in None.
  warn(msg)
/home/runner/miniconda/envs/test-environment/lib/python3.8/site-packages/numpydoc/docscrape.py:377: UserWarning: Unknown section Raises: in the docstring of <function DataFrame.rename at 0x7f381f361e50> in /home/runner/work/koalas/koalas/databricks/koalas/frame.py.
  warn(msg)
/home/runner/miniconda/envs/test-environment/lib/python3.8/site-packages/numpydoc/docscrape.py:377: UserWarning: Unknown section Raises: in the docstring of None in None.
  warn(msg)
/home/runner/miniconda/envs/test-environment/lib/python3.8/site-packages/numpydoc/docscrape.py:377: UserWarning: Unknown section Raises: in the docstring of <function DataFrame.rename at 0x7f381f361e50> in /home/runner/work/koalas/koalas/databricks/koalas/frame.py.
  warn(msg)
/home/runner/work/koalas/koalas/databricks/koalas/plot.py:267: MatplotlibDeprecationWarning: The 'manage_xticks' parameter of bxp() has been renamed 'manage_ticks' since Matplotlib 3.1; support for the old name will be dropped in 3.3.
  artists = ax.bxp(
/home/runner/miniconda/envs/test-environment/lib/python3.8/site-packages/numpydoc/docscrape.py:377: UserWarning: Unknown section Raises: in the docstring of None in None.
  warn(msg)
/home/runner/miniconda/envs/test-environment/lib/python3.8/site-packages/numpydoc/docscrape.py:377: UserWarning: Unknown section Raises: in the docstring of <function DataFrame.rename at 0x7f381f361e50> in /home/runner/work/koalas/koalas/databricks/koalas/frame.py.
  warn(msg)

Warning, treated as error:
don't know which module to import for autodocumenting 'DataFrame.plot' (try placing a "module" or "currentmodule" directive in the document, or giving an explicit module name)
Makefile:21: recipe for target 'html' failed
make: *** [html] Error 2