Fix issue where X-Amz-Content-SHA256 was being set twice

More fallout from the fact that we resign requests.  There
was this Content-SHA256 header that did not have the "delete if
already present" logic, which would cause errors when retrying
S3 sigv4 requests because we would not be signing the requests
properly.

I've added a unittest as well as an integration test to ensure
we don't regress on this in the future.
Fix py3 test issue
Fix flake8 issues
Merge branch 're-fix-sigv4-s3' into develop

* re-fix-sigv4-s3:
  Fix flake8 issues
  Fix py3 test issue
  Fix issue where X-Amz-Content-SHA256 was being set twice
Fix regression for supported timestamp formats

We now accept:

* datetime objects
* ISO8601
* epoch seconds (both as an int or a string)

anytime a timestamp type is needed.  This puts back the previous
behavior that botocore had.

To accomodate this, I've added a new date parsing utility method
in the utils module.  It accepts any of the types listed above and
returns a datetime object with tzinfo.  This makes it easy to write
protocol specific timestamp serializers as they now can assume
they'll always be working with a datetime object with time zone info.
Update service models
Bumping version to 0.76.0
Merge branch 'release-0.76.0' into develop

* release-0.76.0:
  Bumping version to 0.76.0
  Update service models
Merge branch 'timestamp-input' into develop

* timestamp-input:
  Fix regression for supported timestamp formats
Fix integ test failure by specifying a region
Upgrade versions of six/unittest2

Confirmed there are no issues.  Six upgrade is a backwards
compatible change, and we can now use unittest2's latest version,
which we were previously unable to due to unittest2 using a newer
version of six.
Merge branch 'upgrade-six' into develop

* upgrade-six:
  Upgrade versions of six/unittest2
Fix endpoint config for emr

These are based on endpoint prefix.  This means that we were
not creating the proper endpoints for emr, which would fail
SSL cert validation in py2.6.  This was caught by an integration
test that was recently added that happened to use EMR.
Merge branch 'fix-emr-endpoints' into develop

* fix-emr-endpoints:
  Fix endpoint config for emr
Merge branch 'develop' of https://github.com/boto/botocore into develop