Use symlinks when creating virtual environments — fixes #1159

[dairiki]

This changes things to use symlinks when constructing Lektor's private virtual environment.

By default, python -m venv uses symlinks, except when running on Windows (where symlinks can be problematic). However when creating a virtual environment programmatically, venv.EnvBuilder defaults to not using symlinks.

Using symlinks is better (everywhere except maybe on Windows) because:

• It uses less disk space
• That's what python -m venv does
• Apparently, that's what Python on macOS expects, at least when involved via Homebrew (see Lektor cached venv is broken on macOS with Python installed via Homebrew #1159)

Issue(s) Resolved

Fixes #1159

Related Issues / Links

Description of Changes

• Added unit test(s) covering the changes (if testable)