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fixes #7105
fixes #5215
Consolidate all warnings.simplefilter calls to sympy.utilities.exceptions

This is the file where SymPyDeprecationWarning is defined, so the filter will
always be set when that warning is used.

This also changes the warning to once, rather than always, which matches the
default Python warning behavior.

This does cause the SymPyDeprecationWarning to show, which by default it
wouldn't, because Python hides DeprecationWarnings by default.  This is
intentional, as we want our users to know about deprecated behavior.
_test -> _doctest in runtests
do not use fastcache for slow tests
Fix log._eval_is_rational: check for zero

This fixes travis failure in the master:

$ ./bin/test sympy/integrals/tests/test_manual.py --rerun 10
===================================================== test process starts ======================================================executable:         /usr/bin/python  (2.7.3-final-0) [CPython]
architecture:       32-bit
cache:              yes
ground types:       gmpy 1.15
random seed:        41680150
hash randomization: on (PYTHONHASHSEED=3487214350)

sympy/integrals/tests/test_manual.py[17] ..............F..                                                                [FAIL]
_____________________________________________________________________________________________________________________________________________________________________ sympy/integrals/tests/test_manual.py:test_issue_6746 _____________________________________  File "/home/sk/src/sympy/sympy/sympy/integrals/tests/test_manual.py", line 226, in test_issue_6746
    assert manualintegrate((y + 1)**x, x) == x
AssertionError

==================================== tests finished: 16 passed, 1 failed, in 189.46 seconds ====================================DO *NOT* COMMIT!
Merge pull request #8079 from skirpichev/fix-log-is_rational

Fix log._eval_is_rational: check for zero
Merge pull request #8078 from pbrady/no_fastcache_slow

do not use fastcache for slow tests
Merge pull request #8073 from asmeurer/deprecation-warnings

Consolidate all warnings.simplefilter calls to sympy.utilities.exceptions
Merge pull request #8068 from skirpichev/7105-LambertW-branches

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Merge branch 'master' into bound