Make Regression model training set consist only of defects

[marco-c]

Now that we finally have the bug type, we can make the Regressor model differentiate between defects which are regressions and defects which are not regressions, rather than between defects which are regressions and defects which are not regressions + non-defects.

This should make the problem easier and the results better.

Note, we probably have to wait for a bit in order to do this, as currently the bug type is still not extremely precise, as it's not required to fill it yet.

[omegajudith]

maco, am outreachy applicant. am trying to find a python bug on to work on please assign

[Amisha2778]

@marco-c I am an Outreachy applicant. Could you please assign any python related issue?

[suhaibmujahid]

@omegajudith and @Amisha2778 please read #1092.