Single change since v0.7.1: the file-scope coefficient header uses
`inline constexpr double` (and `inline constexpr int`) in place of the
C-era `static const double` / `static const int`.  Same values, same
semantics; one definition per binary (linker-deduped) instead of one per
translation unit; usable in constexpr contexts.  tools/cheby_fit.py's
emit_mathpf generator is updated to match so the next regeneration emits
the same pattern.

Builds now require C++17 (was C++14 in 0.7.1).  setup.py adds /std:c++17
on MSVC and -std=c++17 elsewhere.

No behavioural change; the compiled .pyd/.so binaries embed the same
floating-point constants as 0.7.1.  172/172 tests pass.  (PR #12.)