Renewed support for python3.6+

In line with pyexcel-io v0.6.0.

note: pyexcel-pdfr 0.5.x wont work with pyexcel-io v0.6.0 because the latter demands python 3.6 but the former works only with python 2.

v0.5.0-rc1

Find 0.5.0-rc1 in changelog for more details