Simplified installation with `pip install -r requirements.txt`

[Edward K. Ream]

PR #3775 supports simplified installation using GitHub.

The new (recommended) way to install Leo is to do the following once:

- git clone https://github.com/leo-editor/leo-editor.git

- pip install -r requirements.txt

After that, Leonistas can get the latest code using git pull. It's just that simple.

Legacy installation

Installing/uninstalling Leo using PyPi will work as before:

- pip install leo

- pip uninstall leo

Summary

PR #3775 will soon be part of the devel and 6.7.8 branches.

pip install -r requirements.txt will make installing from GitHub a snap.

Installing Leo from PyPi will work as before.

Edward