Use `python3 -m venv` instead of `virtualenv` to create virtual environment

[valosekj]

After Debian update, fsleyes_preset.py stopped working on one machine due to problems with nibabel dependencies.

I removed the existing venv and tried to create a new one using virtualenv:

$ virtualenv -p /usr/bin/python3 venv
Traceback (most recent call last):
  File "/usr/local/bin/virtualenv", line 7, in <module>
    from virtualenv.__main__ import run_with_catch
  File "/usr/local/lib/python2.7/dist-packages/virtualenv/__init__.py", line 3, in <module>
    from .run import cli_run
  File "/usr/local/lib/python2.7/dist-packages/virtualenv/run/__init__.py", line 6, in <module>
    from virtualenv.run.app_data import AppDataAction
  File "/usr/local/lib/python2.7/dist-packages/virtualenv/run/app_data.py", line 8, in <module>
    from virtualenv.util.lock import ReentrantFileLock
  File "/usr/local/lib/python2.7/dist-packages/virtualenv/util/lock.py", line 11, in <module>
    from virtualenv.util.path import Path
  File "/usr/local/lib/python2.7/dist-packages/virtualenv/util/path/__init__.py", line 3, in <module>
    from ._pathlib import Path
  File "/usr/local/lib/python2.7/dist-packages/virtualenv/util/path/_pathlib/__init__.py", line 5, in <module>
    import six
ImportError: No module named six

However, this failed. Thus I tried another way how to create venv:

python3 -m venv venv

Then I activated the venv and installed dependencies (manual here). Now, fsleyes_preset.py works again.