Fix pyenv FAQ env var commands

When using pyenv and global not set (defaulting to system) or global set to `system` the Launcher was unable to determine correct version to use.

Fixed by using `pyenv exec` to correctly determine current Python version instead of parsing pyenv config files.

Fixes brettcannon#141

Author: cas--

README.md

@@ -221,20 +221,21 @@ but [U+2502/"Box Drawings Light Vertical"](https://www.compart.com/en/unicode/U+
 ### How can I make the Python Launcher use my default Python version from pyenv?
 
 If you're using [pyenv](https://github.com/pyenv/pyenv) to manage your Python
-versions, you'll want to grab the major and minor version from the first Python
-version listed in your default
-[pyenv version file](https://github.com/pyenv/pyenv#choosing-the-python-version).
+versions, you'll want to set the version the Launcher to the pyenv 
+[global version](https://github.com/pyenv/pyenv/blob/master/COMMANDS.md#pyenv-global).
 
-You can add this line to your `.zshrc` or `.bashrc` file:
+Add these lines to your `.zshrc` or `.bashrc` file:
 
 ```sh
-export PY_PYTHON=$(head -n 1 $(pyenv root)/version | cut -d "." -f 1,2)
+export PY_PYTHON=$(pyenv exec python -c "import sys; print('.'.join(map(str, sys.version_info[0:2])))")
+export PY_PYTHON3=$(pyenv exec python3 -c "import sys; print(*sys.version_info[0:2], sep='.')")
 ```
 
-Or this line in your `~/.config/fish/config.fish` file:
+Or these lines to your `~/.config/fish/config.fish` file:
 
 ```sh
-set -gx PY_PYTHON (head -n 1 (pyenv root)/version | cut -d "." -f 1,2)
+set -gx PY_PYTHON (pyenv exec python -c "import sys; print('.'.join(map(str, sys.version_info[0:2])))")
+set -gx PY_PYTHON3 (pyenv exec python3 -c "import sys; print(*sys.version_info[0:2], sep='.')")
 ```
 
 ## Appendix