Fix arch-specific wheel name, pip install really wants "riscv64" on OpenBSD.
Example:

  ERROR: jellyfish-1.0.3-cp310-cp310-openbsd_7_5_riscv.whl is not a supported wheel on this platform.

Index: src/target.rs
--- src/target.rs.orig
+++ src/target.rs
@@ -130,7 +130,7 @@ impl Arch {
             Arch::Powerpc | Arch::Powerpc64Le | Arch::Powerpc64 => "powerpc",
             Arch::X86 => "i386",
             Arch::X86_64 => "amd64",
-            Arch::Riscv32 | Arch::Riscv64 => "riscv",
+            Arch::Riscv32 | Arch::Riscv64 => "riscv64",
             Arch::Mips64el | Arch::Mips64 | Arch::Mipsel | Arch::Mips => "mips",
             // sparc64 is unsupported since FreeBSD 13.0
             Arch::Sparc64 => "sparc64",
