Isl 0 19

See merge request inducer/islpy!3

Re-Fix: From-string construction: prevent __del__ crash if construction fails (#2 on Gitlab, e02bef63)

Work around pypy incompatibility with distutils.sysconfig

See merge request !1

This reverts commit bd13bed9. Turns out
PwQPolynomial is implicitly zero outside of its domain, and validity
needs to be tracked through other means.

This reverts commit 0a9d94b9.

add isl_schedule_node_type and read_header isl/ilp.h