* nanodet python demo

* add clip

* fix clip wh

* remove nms package requirement

* release python

* fix twine upload only push tags, fix linux bdist_wheel so path error

* fix long_description_content_type

* remove log print

* set cibuildwheel version 1.6.3

* add pyproject.toml

* set release python only on push tag

* add ci build python 2.7

* remove python2.7; update readme.md

* add python
* check elempack when convert ncnn.Mat to numpy.ndarray
fix build with NCNN_STRING=OFF