[Nhk] Fix: NhkVod(|Program) Extractor / [Piksel] Feat: Add variable host support

[menschel]

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• Put an x into all the boxes [ ] relevant to your pull request (like that [x])
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Before submitting a pull request make sure you have:

• Searched the bugtracker for similar pull requests
• Read adding new extractor tutorial
• Read youtube-dl coding conventions and adjusted the code to meet them
• Covered the code with tests (note that PRs without tests will be REJECTED)
• Checked the code with flake8

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What is the purpose of your pull request?

• Bug fix
• Improvement
• New extractor
• New feature

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Description of your pull request and other information

NHK World VOD is broken since a few weeks.

This is due changing to self-hosted API and player instance.

Necessary tweaks were to

• make PikselIE use the host of the given URL instead of hard-coded Piksel host.
• use the correct template url for the initial API call in NhkVodIE

[menschel]

Honestly, I hope this does not result in nitpicking again!
Take it or drop it.

[dirkf]

... again ??

[dirkf]

Thanks for your work!

Looks good: I've made some small suggestions for project conventions and robustness.

[menschel]

Accepted changes.

[menschel]

To say it bluntly, this code review process did not yield any significant quality improvement.
It merely wasted 1 week and introduced a flake8 issue along the way.
Solving this simple duplicate import made me realize how Github has slided down in Quality after Micro$oft has taken over Regime. Please move to another platform.