Fix PythonVirtualenvOperator not working with Airflow context

[feluelle]

Primarly this PR fixes an issue with PythonVirtualenvOperator not working in conjunction with airflow context arg.

Relates to #8177
Relates to #8256

But it also has a QoL improvement. You don't need to manually pass dill as requirement anymore if you have set use_dill to True. So it won't raise an Error if you forgot to add it.

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• Description above provides context of the change
• Unit tests coverage for changes (not needed for documentation changes)
• Target Github ISSUE in description if exists
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cc @maganaluis

[mik-laj]

@feluelle I am a bit busy with other changes. I'm afraid I may not find time to look at it.

@Fokko Can you look at it? I know you are more familiar with this code.

[feluelle]

PTAL @turbaszek @maganaluis @ashb @Fokko

[Fokko]

Looks good, thanks @feluelle

[feluelle]

Ready for another round @turbaszek @Fokko

[turbaszek]

LGTM 🚀 @Fokko @kaxil ?

[feluelle]

@maganaluis if you want to backport this fix to 1.10 branch let me know if I can help you with it. :)

I changed the description so it won't automatically close your issue and PR, because you want to target 1.10.x directly, correct?

[feluelle]

@Fokko @kaxil last chance after 🟢 :)

[maganaluis]

@feluelle Thank you for adding this fix, not sure if this change will work in the 1.10 branch? We forked the repo into GitHub Enterprise and added the fix there, I'm good with closing the issue and PR. I have interest to move to Airflow 2.0 because the REST API so I'm looking forward to it.