Replace control flow diagram with Mermaid

[brettcannon]

https://github.blog/2022-02-14-include-diagrams-markdown-files-mermaid/ means the extra use of Graphviz is unnecessary.

[brettcannon]

https://mermaid-js.github.io/mermaid/#/flowchart

[brettcannon]

graph TD
    start[py ...] --> major_minor[-3.6]
    start --> major_only[-3]
    start --> best[No option specified]

    major_minor --> $PATH

    major_only --> PY_PYTHON3

    $PATH --> exec

    best --> env_var{$VIRTUAL_ENV}
    
    env_var --> venv_dir{.venv}
    
    venv_dir --> shebang{#! ...}
    venv_dir --> exec
    
    shebang --> PY_PYTHON([$PY_PYTHON])
    shebang --> PY_PYTHON3([$PY_PYTHON3])

    PY_PYTHON3 --> $PATH
    PY_PYTHON --> $PATH
    shebang --> $PATH

    env_var --> exec[Execute Python]

[brettcannon]

Tracking in #180 .