Removed the slow tests which are never actually run to completion on
travis due to timeout errors.  Those tests and there run times are:

6745.05s call     sympy/physics/mechanics/tests/test_kane3.py::test_bicycle
2083.29s call     sympy/solvers/tests/test_ode.py::test_1st_exact2
1522.66s call     sympy/integrals/tests/test_failing_integrals.py::test_issue_4540
1221.33s call     sympy/core/tests/test_wester.py::test_W25
1090.84s call     sympy/integrals/tests/test_failing_integrals.py::test_issue_4941
963.31s call     sympy/solvers/tests/test_solvers.py::test_issue_6528
437.72s call     sympy/integrals/tests/test_heurisch.py::test_pmint_WrightOmega
411.27s call     sympy/core/tests/test_wester.py::test_W24
PEP8 fixes for euler_equations
Export euler_equations
Change output format to list, cleanup test_euler.py
Improve docstring of the euler_equation function
euler_equations: funcs/vars arguments support now arbitrary iterables
clear the cache once per file rather than after every test with pytest
Refactor setup_module methods out of sympy tests and use a single module level
fixture in conftest.py instead
MatExpr: Fix entry when power is zero and other cases

 *  `MatPow(A, 0)[0, 0]` would give an error because an empty list was
    passed to MatMult.  Fixed, added tests.
 *  Entries of `MatPow` of ImmutableMatrix added as a special case.
    This fixes `.as_explicit` for MatPow of ImmutableMatrices, added
    tests for this.
 *  Previously, `MatPow._entry` would return None if couldn't do it,
    instead raise errors.
 *  Negative powers probably do-able in some cases, left comments
    for future improvement.
Reformat some references
Add sphinx documentation for old assumptions in the core
Implementing the Rademacher distribution
Ensure SYMPY_DEBUG env variable contains only True or False

Evaluating it unchecked is an unlikely yet possible vector of attack,
e.g. one can gain root privileges if there's a script importing sympy
with setuid bit on it.
8046;16.1-3: enhance and correct Add._eval_is_(positive|negative)

The checks for a.is_zero were removed because those are already
handled outside of the loop.
8046;17: Add._eval_is_imaginary modifications
Merge pull request #7936 from skirpichev/old-assump-docs

Add sphinx documentation for old assumptions in the core
Merge pull request #8142 from skirpichev/euler2

Improvements for euler_equations
Merge pull request #8146 from immerrr/strict-eval-of-sympy-debug

Don't evaluate arbitrary text coming from SYMPY_DEBUG variable
Merge pull request #8162 from smichr/8046

Add._eval_is_(pos, neg, imag) changes
correct issue numbers