Deduplicate module initialization

[peytondmurray]

This PR patches the pyodbc module initialization, making npyodbcmodule.cpp the only place where PyInit_<module_name> is called.

Other changes

• As an additional step towards Code documentation #11, we also move the compiled extension into a submodule, at npyodbc._npyodbc. As a result of this choice, we now also run the CI tests from inside tests/ directly; otherwise pytest will add the root of the repo to sys.path (yes, even without a tests/__init__.py and with --import-mode=importlib) and thus inadvertently try to import the local npyodbc rather than the one installed at site-packages, causing it to fail to find the compiled extension.
• Old tests were removed; they can't be run anyway due to some missing modules.
• Add missing -Wno-write-strings cflag for Unix builds to suppress compiler warnings

Closes #63.

[peytondmurray]

See mkleehammer/pyodbc#1450 for upstream segfault discussion.

A null-commit applied to pyodbc==5.3.0 generates the same segfault: peytondmurray/pyodbc#1, so I don't consider these test failures as blocking.