Release: don't build Python 3 tarball

Just build one tar.gz, it should work with all Pythons.
Document the logx parameter in Expr.nseries()

Add doctests. This parameter was introduced in
9c46974da12df56818020e29f76313fc0903ecc6 but it was never documented.
Fix heuristics for real, bounded symbols, for example: limit(r*sin(1/r), r, 0)
More cleanup in heuristics()
Fix fact extraction from composite assumptions.

_extract_facts is only passed expressions in CNF and returns None
when it can't find a meaningful fact to extract.

This is enough to guarantee a correct handling of all possible forms of
assumptions and allows the following to work correctly:

    assert ask(Q.real(x), Q.real(x) & Q.real(y)) is True
    assert ask(Q.real(x), Q.real(x) | Q.real(y)) is None
    assert ask(Q.real(x), ~(Q.real(x) >> Q.real(y))) is True
Fix tests for composite assumptions, add new ones in test_assumptions_2.py/test_query.py
Fix typo
Call simplify() in Add._eval_as_leading_term.  Fix issue 3265.
Extract nested Order expr
Handle NotImplementedError in Pow.nseries
Raise TypeError while doing comparison of complex numbers (issue 2625)
Add tests for issue 2625
Fix test failure in test_zeta_functions.py
Merge branch 'master' into 3265-eval-lt-fix
Add numeric computation doc page

Discusses lambdify, ufuncify, and Theano options for numeric evaluation
add link to numeric computation in modules index
replace x.length with n
add evalf
Merge branch 'master' into 2625-complex-comparison
A better fix for test failure in test_zeta_functions.py (commit 80fe207)