Xapian2 support

[ojwb]

The existing CI on master seems to have bitrotted and fails if re-run now without changing the repo contents at all. I've fixed some of the problems, but I don't understand why one of the 3.8 builds fails, nor why all the 3.10 builds fail, so I've just excluded those. See individual commit messages for details of the errors I ran into. As I said in #239, I don't develop (or even use) django, haystack or xapian-haystack, and I do very little Python, so I think I'm the wrong person for the job of trying to fix these excluded builds.

The key changes which are actually updating the code for Xapian 2 are in 2bebec9:

    Eliminate use of xapian.ValueRangeProcessor
    
    This class was deprecated in Xapian 1.4 and removed in Xapian 2.
    Instead use xapian.RangeProcessor or construct Query objects directly.
    
    This makes xapian-haystack compatible with Xapian 2.  Fixes #239.
    
    Open-ended ranges now build open-ended Xapian range queries, addressing
    two TODO comments.
    
    A range query on an unknown field is now handle gracefully.  Fixes #236.
    
    A range query of type int or float with an open upper end no longer
    requires `*` for the upper end.  Fixes #217.

The Xapian 1.5.1 version used in CI is a release candidate for 2.0.0 so that should be adjusted to use 2.0.0 once actually released.

[coveralls]

coverage: 97.376% (-0.2%) from 97.588%
when pulling 690f45f on ojwb:xapian2-support
into 5eec1ff on notanumber:master.

[ojwb]

The Xapian 1.5.1 version used in CI is a release candidate for 2.0.0 so that should be adjusted to use 2.0.0 once actually released.

2.0.0 is now released so I've pushed an update to test with it.

[claudep]

Hi Olly, I just pushed a separate PR where we fixed the test suite and upgraded to recent Pthon/Django versions. May I ask you to rebase your PR on latest master? You should be now able to concentrate to xapian 2 compat only. Sorry for not doing that sooner.

[asedeno]

The decrease in coverage technically correct, but a false positive. It comes from dropping the special case for odd minor versions, which reduced the line count by two and the covered line count by one.

[claudep]

@asedeno Does your comment implies you are approving this PR?

[asedeno]

I have not reviewed the changes in detail, but the wheels build and the tests pass, which satisfies me as a former xapian-haystack user. Note that I am no longer at the job where I was using this, so I haven't actually used this in years.

[asedeno]

@asedeno Does your comment implies you are approving this PR?

@claudep I've looked over this some more. Yes, we should merge this.

[claudep]

@ojwb, as we will probably make a release soon after merging this patch, it would be great to have some content for the Changelog related to these changes. Would you agree to suggest something?

[ojwb]

I already summarised the changes in the first comment:

    Eliminate use of xapian.ValueRangeProcessor
    
    This class was deprecated in Xapian 1.4 and removed in Xapian 2.
    Instead use xapian.RangeProcessor or construct Query objects directly.
    
    This makes xapian-haystack compatible with Xapian 2.  Fixes #239.
    
    Open-ended ranges now build open-ended Xapian range queries, addressing
    two TODO comments.
    
    A range query on an unknown field is now handle gracefully.  Fixes #236.
    
    A range query of type int or float with an open upper end no longer
    requires `*` for the upper end.  Fixes #217.