Key pickling 2.0: reverse unused

[jquast]

This builds on PR #30, retuning coverage to 100% and removing unused traceback handlers.

• spellfix compatibile => compatible
• test_sqlite_migration.py was writing files to ./test/db, not ./tests/db.
• use sys.version_info < (3, 0) rather than tracking _major_version.
• Introduce 'unittest2' as development dependency, removing the TestCaseBackport class.
• No longer necessary to use 'reraise', track inner or outer stack, as the next release of sqlitedict always pickles and there is no known exception to report.
• SqliteDict.str depended on self.conn.filename always being set, but this is not necessarily True, (esp. in test case teardown due to del).
• add missing test coverage for throwing KeyError when any key or value cannot be unpickled.

[piskvorky]

@jquast I see the exception handling code has been removed in this PR. How vital was the code? Is it needed or not?

I'd obviously prefer to have it removed, it obfuscates the code greatly. But I don't remember how critical it is, so I'm afraid to touch it. Can you comment?

(I'm only talking about the inner/out stacks, not key encoding here)

[menshikh-iv]

Hi @jquast @ziky90, are you planning to continue this work?

[menshikh-iv]

This PR looks abandoned, for this reason, I close it.