title: NameError when starting NeoVim while Python virtual environment is active
body: <p>When I launch NeoVim while a virtualenv is activated, I get the following error:</p>
  
  <pre><code>Error detected while processing function provider#python3#Call:
  line   18:
  Error invoking 'python_execute' on channel 3 (python3-script-host):
  Traceback (most recent call last):
    File "&lt;string&gt;", line 6, in &lt;module&gt;
  NameError: name 'execfile' is not defined
  Press ENTER or type command to continue
  </code></pre>
  
  <p>This is what I see when I run <code>:checkhealth provider</code>:</p>
  
  <pre><code>
  health#provider#check
  ========================================================================
  ## Clipboard (optional)
    - OK: Clipboard tool found: pbcopy
  
  ## Python 2 provider (optional)
    - INFO: pyenv: Path: /usr/local/Cellar/pyenv/1.2.16/libexec/pyenv
    - INFO: pyenv: $PYENV_ROOT is not set. Infer from `pyenv root`.
    - INFO: pyenv: Root: /Users/myname/.pyenv
    - INFO: Using: g:python_host_prog = "~/.pyenv/versions/2.7.16/envs/neovim-python2-venv/bin/python"
    - INFO: $VIRTUAL_ENV matches executable
    - INFO: Executable: /Users/myname/.pyenv/versions/2.7.16/envs/neovim-python2-venv/bin/python
    - INFO: Python version: 2.7.16
    - INFO: pynvim version: 0.4.1
    - OK: Latest pynvim is installed.
  
  ## Python 3 provider (optional)
    - INFO: pyenv: Path: /usr/local/Cellar/pyenv/1.2.16/libexec/pyenv
    - INFO: pyenv: $PYENV_ROOT is not set. Infer from `pyenv root`.
    - INFO: pyenv: Root: /Users/myname/.pyenv
    - INFO: Using: g:python3_host_prog = "~/.virtualenvs/neovim-python3-venv/bin/python3"
    - WARNING: $VIRTUAL_ENV exists but appears to be inactive. This could lead to unexpected results.
      - ADVICE:
        - If you are using Zsh, see: http://vi.stackexchange.com/a/7654
    - INFO: Executable: /Users/myname/.virtualenvs/neovim-python3-venv/bin/python3
    - INFO: Python version: 3.8.0
    - INFO: pynvim version: 0.4.1
    - OK: Latest pynvim is installed.
  </code></pre>
  
  <p>I don't use Zsh, I use Bash. An attempt to use the snippet from the linked answer in <code>.bashrc</code> did not change the situation.</p>
  
  <p>I do not get the error when not launching NeoVim while a virtualenv is activated.</p>
  
  <p>Removing all Vim plugins did not get rid of the error.</p>
  
  <p>I have tried this with virtualenvs that use both Python 2 and Python 3. The error is the same.</p>