add virtualenv files to .gitignore

[rvalyi]

when working with virtualenv (possibly for local dev), one might want to create a project specific virtualenv.
To mimic how it's done in the base Bedrock Acsone image, one might then create it in the odoo folder [1]:

python -m virtualenv odoo

When this is done, the virtualenv files would suddenly pollute the git repo/git status. This .gitignore change avoids that.

[1] https://github.com/acsone/odoo-bedrock/blob/master/Dockerfile-16.0#L35