patch time.time in historical_connections to avoid windows failures; fix race condition in pool.availableGC

* patch time.time in historical_connections to avoid windows failures.

* Also use a mock version of datetime.datetime.utcnow.

We can't patch the method in-place because it's an extension type.

* synchronize sleep with time

* increment by less than the pool timeout to avoid a race condition.

* Avoid mutating the dict as we iter over it.

Seen in Python 3.4 tests:

File "/home/travis/build/zopefoundation/ZODB/src/ZODB/historical_connections.txt", line 263, in historical_connections.txt
Failed example:
    conn = db.open()
Exception raised:
    Traceback (most recent call last):
      File "/opt/python/3.4.2/lib/python3.4/doctest.py", line 1324, in __run
        compileflags, 1), test.globs)
      File "<doctest historical_connections.txt[0]>", line 1, in <module>
        conn = db.open()
      File "/home/travis/build/zopefoundation/ZODB/src/ZODB/DB.py", line 761, in open
        self.historical_pool.availableGC()
      File "/home/travis/build/zopefoundation/ZODB/src/ZODB/DB.py", line 277, in availableGC
        for key, pool in self.pools.items():
    RuntimeError: dictionary changed size during iteration

Do3cc fix for45 master

Nested connections try to clean up themselves multiple
times. This does not work

Drop support for Python 2.6 / 3.2.

3.2 is no longer supported by our packaging / CI tools.

2.6 is long out-of-support from Python devs.

Cleanup tox.ini and .travis.yml as mentioned in #42 (and Python 3.5)

- (Both) Remove repetition of deps. Only install tests_require deps
  explicitly. The previous comment in tox.ini about eggs in the
  working directory doesn't apply, per @mgedmin.
- Setup Travis to use the local wheel cache for faster install times.

See http://zestreleaser.readthedocs.org/en/latest/versions.html.

Make zodbpickle non-optional. 

(IIRC a bug in python-modernize causes duplicate future imports)

Use PEP-426 environment markers

Any attempts to use platform_python_implementation or
implementation_name result in an "Invalid environment marker" error.

Any attempts to use >= to compare python_version result in an "Invalid
environment marker" error.

Augh documentation for all of this is painfully suboptimal.

Should fix #36

log ConflictError details, fixes #29

Update issue #22

Incorporate feedback and make the tests run under Python3. Notably, the Grand Exception Unification in Python 3.3 means that IOError is a synonym for OSError in those versions, but not before; deal with this by catching both types and throwing OSError in the doctests (because that's how it prints). Fix #21. Fix #22.

Fix #21, FileStorage: when disk space runs out while packing, do not leave the ``.pack`` file around

fixed ResolveableWhenStateDoesNotChange, changed show_tryToResolveConflict_log_output to check both cases

Support PyPy. 

This is a performance optimization for PyPy (profiling shows many recursive calls to Persistent.__getattribute__, and recursive calls seem to trouble the PyPy JIT; this eliminates many of them when loading objects):

"Transaction",                 AFTER       BEFORE
"Add 3000 Objects",             20205      18505
"Update 3000 Objects",          24191      18807
"Read 3000 Warm Objects",       28920      26671
"Read 3000 Cold Objects",       28745      27487
"Read 3000 Hot Objects",        29563      27487
"Read 3000 Steamin' Objects", 1033758     916587