* quantize and dequantize tests

* unify activation and usability function

* drop NCNN_REQUANT cmake option, test dequantize requantize pack8, fix webassembly build

* benchmark use requantize int8 model

* use local pool allocator

* detach extract feat from local allocator

* fix test

* add test python vulkan

* add python vulkan ci on linux with 3.6 and 3.8

* fix ci vulkan icd

* nanodet python demo

* add clip

* fix clip wh

* remove nms package requirement

* fix model zoo download files

* fix yolov4.bin download split

* fix python/examples/shufflenetv2

* 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

Co-authored-by: Cai Shanli <caishanli25@gmail.com>

* add pytest net

* fix typo error

* fix header year

* Update README.md

type in pip command to install requirements

* Update model_store.py

fix not working url. the models in the previous url were just stub.

add test mat, blob and paramdict
fix README.md build
print mat'data in python

fix build python with lastest ncnn

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