Fix compilation warnings

[thomasst]

Variable declaration should be wrapped in #ifdef for Python versions where it's not needed (I believe >= 3.2).

module.c: In function ‘PyInit_ciso8601’:
module.c:414:15: warning: unused variable ‘pytz’ [-Wunused-variable]
     PyObject *pytz;

Also, should we trigger errors on compile warnings to catch these?

[movermeyer]

There are several compilation warnings right now, depending on the Python version:

Python 2.7

module.c: In function ‘_parse’:
module.c:126:13: warning: format ‘%d’ expects argument of type ‘int’, but argument 5 has type ‘long unsigned int’ [-Wformat=]
             PARSE_SEPARATOR(IS_CALENDAR_DATE_SEPARATOR, "date separator ('-')")
             ^
module.c:205:9: warning: format ‘%d’ expects argument of type ‘int’, but argument 5 has type ‘long unsigned int’ [-Wformat=]
         PARSE_SEPARATOR(IS_DATE_AND_TIME_SEPARATOR,
         ^
module.c:223:21: warning: format ‘%d’ expects argument of type ‘int’, but argument 5 has type ‘long unsigned int’ [-Wformat=]
                     PARSE_SEPARATOR(IS_TIME_SEPARATOR, "time separator (':')")
                     ^
module.c: In function ‘initciso8601’:
module.c:415:15: warning: unused variable ‘datetime’ [-Wunused-variable]
     PyObject *datetime;

Python 3.7

env37/bin/python setup.py bdist
...
module.c: In function ‘PyInit_ciso8601’:
module.c:415:15: warning: unused variable ‘datetime’ [-Wunused-variable]
     PyObject *datetime;
               ^
module.c:414:15: warning: unused variable ‘pytz’ [-Wunused-variable]
     PyObject *pytz;
               ^
module.c: At top level:
module.c:4:18: warning: ‘fixed_offset’ defined but not used [-Wunused-variable]
 static PyObject *fixed_offset;
                  ^

Python 3.6 on Windows:

module.c
module.c(107): error C2220: warning treated as error - no 'object' file generated
module.c(107): warning C4267: 'function': conversion from 'size_t' to 'int', possible loss of data
module.c(127): warning C4267: 'function': conversion from 'size_t' to 'int', possible loss of data
module.c(133): warning C4267: 'function': conversion from 'size_t' to 'int', possible loss of data
module.c(141): warning C4267: 'function': conversion from 'size_t' to 'int', possible loss of data
module.c(145): warning C4267: 'function': conversion from 'size_t' to 'int', possible loss of data
module.c(213): warning C4267: 'function': conversion from 'size_t' to 'int', possible loss of data
module.c(223): warning C4267: 'function': conversion from 'size_t' to 'int', possible loss of data
module.c(230): warning C4267: 'function': conversion from 'size_t' to 'int', possible loss of data
module.c(235): warning C4267: 'function': conversion from 'size_t' to 'int', possible loss of data
module.c(241): warning C4267: 'function': conversion from 'size_t' to 'int', possible loss of data
module.c(245): warning C4267: 'function': conversion from 'size_t' to 'int', possible loss of data
module.c(250): warning C4267: 'function': conversion from 'size_t' to 'int', possible loss of data
module.c(295): warning C4267: 'function': conversion from 'size_t' to 'int', possible loss of data
module.c(300): warning C4267: 'function': conversion from 'size_t' to 'int', possible loss of data
module.c(303): warning C4267: 'function': conversion from 'size_t' to 'int', possible loss of data

There are also 2 warnings from Python about missing metadata in setup.py:

warning: check: missing required meta-data: url

warning: check: missing meta-data: either (author and author_email) or (maintainer and maintainer_email) must be supplied

I'll create a PR to fix all these. The code doesn't get any cleaner, but it might be nice to be able to fail the CI on new warnings.