Drop usage of private httpx ContentStreams

[tomchristie]

Rejig the auth flow slightly, so as to not rely on httpx's private ContentStreams.

After this fix I believe authlib should be compatible with httpx 0.12, the upcoming 0.13, and the planned 1.x series.

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

(If no, please delete the above question and this text message.)

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[lepture]

Thanks