fix: add spec version header to polling connector requests

[Tymek]

Description

https://linear.app/unleash/issue/1-4062/python-sdk-should-send-spec-headers

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes.

• Unit tests
• Spec Tests
• Integration tests / Manual Tests

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[coveralls]

Pull Request Test Coverage Report for Build 17494148669

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 93.939%

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Totals
Change from base Build 17488384346:	0.0%
Covered Lines:	620
Relevant Lines:	660

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💛 - Coveralls