How do I switch betwen Python 2 and 3 when using Flycheck with flake8 in Emacs?

How do I switch betwen Python 2 and 3 when using Flycheck with flake8 in Emacs?

Having installed Flycheck for Emacs, I then install a syntax-checker like flake8.

Installing flake8 using pip install flake8 makes Flycheck check Python 2 syntax.

However, using pip3 install flake8 makes Flycheck check the syntax for Python 3.

How can I switch between the modes for Python 2 and 3 in Emacs 24?