Windows "missing DLL" fix

A backwards compatibility bug on Windows was causing "missing DLL" errors and not allowing pyodbc to be loaded (#663). This affected Python versions 3.5+. This was due to building with a compiler that was newer than those used by the official Python builds. The newer Visual Studio 2019 introduced a dependency a new DLL which doesn't ship with Python, so pyodbc would fail to load unless something else had already installed the DLL.

Keith Erskine added a fix to check for overflow of integer and floating point parameters so that a proper Python exception is raised (#671).

Kieth and Gord Thompson also provided a bunch of improvements for unit tests.

Anatoli Babenia updated the readme with a badge for PyPI which provides a quick link to it. Very nice. PyPI should now also include the contents of the readme instead of just a single sentence thanks to a commit by Keith.