Fix PR attributes handling

[joao-p-marques]

body_html and body_text attributes in PRs were removed. Only body exists now

@Tecnativa
TT21199

[sigmavirus24]

This appears to be completely incorrect based on very minimal testing:

>>> r = gh.repository('sigmavirus24', 'github3.py')
>>> pr = next(r.pull_requests())
>>> pr
<Short Pull Request [#1048]>

>>> short_pr = pr
>>> pr = short_pr.refresh()
>>> pr
<Pull Request [#1048]>

>>> pr.body
'`body_html` and `body_text` attributes in PRs were removed. Only `body` exists now\r\n\r\n@Tecnativa\r\nTT21199'

>>> pr.body_html
'<p><code>body_html</code> and <code>body_text</code> attributes in PRs were removed. Only <code>body</code> exists now</p>\n<p><a class="user-mention" data-hovercard-type="organization" data-hovercard-url="/orgs/Tecnativa/hovercard" href="https://github.com/Tecnativa">@Tecnativa</a><br>\nTT21199</p>'

>>> pr.body_text
'body_html and body_text attributes in PRs were removed. Only body exists now\n@Tecnativa\nTT21199'

>>>

This library uses the full media type in https://docs.github.com/en/rest/reference/pulls#custom-media-types-for-pull-requests which sends all 3 attributes

[joao-p-marques]

@sigmavirus24 Ok, sorry, I ran into this error but it was indeed a problem between the chair and the keyboard 😅

Thanks for pointing to the correct media type, that solved the issue!