[FEATURE]: skip `type="text/javascript"` attribute in `<script>` tag of HTML output

[hannob]

Description

plotly.py's HTML output of the write_html() function currently adds a type attribute to the script tag, which is discouraged and causes a warning in the HTML validator:
<script type="text/javascript">

Why should this feature be added?

Standard Javascript code is the default for the <script> tag in HTML, adding a type attribute to declare that default is unnecessary.
The current HTML5 spec explicitly discourages setting it:
"Omitting the attribute, setting it to the empty string, or setting it to a JavaScript MIME type essence match means that the script is a classic script, to be interpreted according to the JavaScript Script top-level production. Authors should omit the type attribute instead of redundantly setting it."

Source: https://html.spec.whatwg.org/multipage/scripting.html#attr-script-type

[robertclaus]

@hannob this is a great callout. This might be a very simple change in the codebase if you are interested in opening a PR for it.