trac #15521: Deprecations: default LP variables will become real instead of nonnegative
improve support for PARI power series
Merge branch 'ticket/15549-improve_gen_construction' into pari_series
check for exact 0 in series substitution
trac #15333 remove import of is_* in modules/all.py
trac #15333 remove global imports in structure/all.py
Merge branch 'develop' into ticket/15599-pari_series
trac #15521: Rebase on 6.1.beta6
trac #15521: Fixing the docstests reported by the patchbot
Fix bugs in partial_fraction_decomposition.

As implemented, the partial fraction decomposition did not reduce
beyond irreducable powers, as pointed out in ticket #15306.

In fixing this I also uncovered a more serious bug, namely the
(non-unique) choice of remainder in the integer case can lead
to the "whole" part being off from numer // denom.  This has been
fixed.
Larger example.
trac #15521: Rebase on 6.1.rc1
trac #15521: A typo
Add optional argument to fraction decomposition, more docs.
Fix doctests (they're equivalent to the previous output).
Add overflow check to divisors()
#15306 rev. patch: simplifications, more tests
Added reduced Burau representation, Alexander polynomial to Braid class.
trac #15843 details, minor changes
trac #15843 make sure tests pass