Add pytest dependency to development requirements
Add pytest fixture for dbs and migrate various tests

This adds a pytest fixture for a temporary database. The other changes
are just the removal of the old context manager, and the reduction of an
indentation level.

(Use 'git show --ignore-all-space' to see the relevant portions.)
Replace all assert_equal() with normal asserts
Switch argument order in equality assertions
Translate all nose assert helpers to normal assertions
Translate test skipping to pytest semantics
Cleanup Plyvel imports in tests

Use a single import instead of also importing plyvel.DB separately.
Drop nose completely
Cleanup Python 2/3 compatibility code in tests
Use tox for testing against multiple Python versions
PEP8 cleanup
Fix tests for Python 2.6
Do not specify the directory for test databases

Just let the 'tempfile' module handle that.
Migrate most "tmp_db" usages to pytest fixtures
Migrate remaining tmp_db() calls to pytest fixtures
Add py32 to tox.ini
Drop pytest-cov from tox.ini; it's unused
Expand ignore patterns
[merge] Migrate test infrastructure from nose to pytest and tox