Fix missing method str.removeprefix in Python 3.8

[bartbroere]

Eland still supports Python 3.8, so str.removeprefix can be called and raise an AttributeError.

Once Eland drops Python 3.8 support this if/else can be removed (only keeping the code in the if).

[bartbroere]

I would use the slice and mention this can be replaced by removeprefix when we remove Python 3.8.

That sounds a bit less verbose indeed. I committed the suggestion.

[pquentin]

buildkite test this please

[pquentin]

buildkite test this please

[pquentin]

buildkite test this please