Add selection of Pypi mirrors in the GUI

### 📝 Description of the feature

In some countries (e.g. China), access to Pypi is a bit slow, especially when downloading huge modules like Tensorflow and other ML modules.

An efficient solution is to setup a mirror site for Pypi. For example in China you can use `https://pypi.tuna.tsinghua.edu.cn`.

It would be nice to have a way to set-up a mirror, and even nicer to have a pre-polulated list.

Also, some users might use a private Pypi server in their company, which uses the same concept

### 💡 Steps for implementing the feature

* [ ] Find a list of the best pypi mirror servers
* [ ] Update the UI to be able to select one and add your own
* [ ] Update the backend to launch `pip` with this server

### 🔗 Useful links and references

To use pip with a mirror:
`pip install -i https://pypi.tuna.tsinghua.edu.cn/simple some-package`

To set as default:
`pip config set global.index-url https://pypi.tuna.tsinghua.edu.cn/simple`

Documentation of 2 mirrors (both in China):
* https://mirror.tuna.tsinghua.edu.cn/help/pypi/
* https://mirrors.sustech.edu.cn/help/pypi.html#_2-configure-index-url