utils.c functions definition moved out of psycopg.h

Some utility functions defined into psycopgmodule.c moved into utils.c.

Currently behaving exactly like the previous (named)tuple.

Drop legacy distutils fallback in setup.py

distutils is not recommended for use and unnecessary for modern Python
environments. Use only setuptools instead. From
https://docs.python.org/3/library/distutils.html:

> Most Python users will not want to use this module directly, but
> instead use the cross-version tools maintained by the Python Packaging
> Authority. In particular, setuptools is an enhanced alternative to
> distutils ...
>
> The recommended pip installer runs all setup.py scripts with
> setuptools, even if the script itself only imports distutils. Refer to
> the Python Packaging User Guide for more information.

pyscopg2 only supports postgres >= 9.1 these days. Thus, taking this fallback will only lead to failure later down the line.

Python 3.7 was released on June 27, 2018.

https://docs.python.org/3/whatsnew/3.7.html

Use a macro trick to add the quotes. This seems more portable than
passing the quotes to the command line (see #658).

https://gcc.gnu.org/onlinedocs/cpp/Stringizing.html