* refactor check_pr_approval, allow using github login-id

2. remove the tricks for paddle.fluid.layers.ops.func

* add testcases

* simplify the test data, and added to file diff approvals

* remove a approver

* test_print_signatrues runs on a simple pipeline, no paddle installed

* testcases for print_signatrures and sampcd . python3 only.

* remove unused import directives

* remove unused import directives

* update testcases

* remove unused codes

* update the docstring for samcd_processor

* no need to import the six module

* 我也不知道为何有一个前导空格，但现在有单元测试，取消这个空格没啥问题

* add unittests for print_signatures; add the first case for 'required' mechanism when executing sample code testing

* there is no paddle installed in PR-CI-APPROVAL

test=document_fix