Removed seq field from Header messages as they are no longer supported in ROS2

[iliabaranov]

As mentioned here: https://github.com/gramaziokohler/roslibpy/issues/109

Header message in ROS2 no longer supports the seq field. Hence I have removed it, and modified the tests to ensure the correct behavior is followed.

Followed the steps here: https://github.com/gramaziokohler/roslibpy/blob/f60cf8f3c385985e59bbb8967caf6ec5c8355c3d/CONTRIBUTING.rst

Unsure this is a breaking change? I suspect so, but really it makes the header message work properly now, where it failed before.

What type of change is this?

• [ ] Bug fix in a backwards-compatible manner.
• [ ] New feature in a backwards-compatible manner.
• [X] Breaking change: bug fix or new feature that involve incompatible API changes.
• [ ] Other (e.g. doc update, configuration, etc)

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

• [ X] I added a line to the CHANGELOG.rst file in the Unreleased section under the most fitting heading (e.g. Added, Changed, Removed).
• [X] I ran all tests on my computer and it's all green (i.e. invoke test).
• [X] I ran lint on my computer and there are no errors (i.e. invoke check).
• [X] I have added tests that prove my fix is effective or that my feature works.