Update .gitignore for current code layout and venv usage

[echoix]

The .gitignore has not been updated since the code layout has been changed a couple of years ago. For example, the pattern for .egg-info assumed the code was in ply/, and now is in src/ply/. In the same time frame, the makefile was tuned to create a venv, but wasn't ignored in the .gitignore.

This PR addresses these two points, by ignoring *.egg-info in any subfolder, and also ignoring the .venv folder used.