代码退场，清除了与python2相关的一些条件分支

[caolonghao]

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来源自 Call for contribution - Python 2.7 相关代码退场

代码退场，清除了 rnn layer, unittests, pslib 等代码中一些 python2 相关的条件分支

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[SigureMo]

有一些小建议(๑>؂<๑），此外貌似还有两个文件没有修改

Paddle/python/paddle/tests/test_callback_visualdl.py

Lines 51 to 52 in dc3b938

	if sys.version_info < (3, ):
	return

Paddle/python/paddle/fluid/param_attr.py

Lines 87 to 90 in dc3b938

	if sys.version_info.major == 2:
	check_type(name, "name", (str, type(None), unicode), "ParamAttr")
	else:
	check_type(name, "name", (str, type(None)), "ParamAttr")

[SigureMo]

这里的 PR-CI-Coverage 覆盖率不过与本 PR 无关，是否可以豁免掉？

[luotao1]

已经豁免