chore(hugr-py): Swap zstd dependency for pyzstd (#2038)

Both libraries offer good support for encoding/decoding zstd, but `zstd`
fails to build under PEP517.

Some poetry error hints:
```
Note: This error originates from the build backend, and is likely not a problem with poetry but with zstd (1.5.6.6) not supporting PEP 517 builds. You can verify this by running 'pip wheel --no-cache-dir --use-pep517 "zstd (==1.5.6.6)"
```

As the libraries are almost interchangeable, let's try using the other
one.

---------

Co-authored-by: Douglas Wilson <141026920+doug-q@users.noreply.github.com>

Author: aborgna-q

hugr-py/pyproject.toml

@@ -32,7 +32,7 @@ dependencies = [
     "pydantic-extra-types>=2.9.0",
     "semver>=3.0.2",
     "typing-extensions~=4.12",
-    "zstd>=1.5.6.6",
+    "pyzstd~=0.16.2",
 ]
 
 [project.optional-dependencies]

hugr-py/src/hugr/envelope.py

@@ -36,7 +36,7 @@
 from enum import Enum
 from typing import TYPE_CHECKING, ClassVar
 
-import zstd
+import pyzstd
 
 if TYPE_CHECKING:
     from hugr.package import Package
@@ -62,7 +62,7 @@ def make_envelope(package: Package, config: EnvelopeConfig) -> bytes:
             raise ValueError(msg)
 
     if config.zstd is not None:
-        payload = zstd.compress(payload, config.zstd)
+        payload = pyzstd.compress(payload, config.zstd)
 
     envelope += payload
     return bytes(envelope)
@@ -85,7 +85,7 @@ def read_envelope(envelope: bytes) -> Package:
     payload = envelope[10:]
 
     if header.zstd:
-        payload = zstd.uncompress(payload)
+        payload = pyzstd.decompress(payload)
 
     match header.format:
         case EnvelopeFormat.JSON:

hugr-py/tests/test_envelope.py

@@ -0,0 +1,32 @@
+from hugr import tys
+from hugr.build.function import Module
+from hugr.envelope import EnvelopeConfig, EnvelopeFormat
+from hugr.package import Package
+
+
+def test_envelope():
+    mod = Module()
+    f_id = mod.define_function("id", [tys.Qubit])
+    f_id.set_outputs(f_id.input_node[0])
+
+    mod2 = Module()
+    f_id_decl = mod2.declare_function(
+        "id", tys.PolyFuncType([], tys.FunctionType([tys.Qubit], [tys.Qubit]))
+    )
+    f_main = mod2.define_main([tys.Qubit])
+    q = f_main.input_node[0]
+    call = f_main.call(f_id_decl, q)
+    f_main.set_outputs(call)
+    package = Package([mod.hugr, mod2.hugr])
+
+    # Binary compression roundtrip
+    for format in [EnvelopeFormat.JSON]:
+        for compression in [None, 0]:
+            encoded = package.to_bytes(EnvelopeConfig(format=format, zstd=compression))
+            decoded = Package.from_bytes(encoded)
+            assert decoded == package
+
+    # String roundtrip
+    encoded = package.to_str(EnvelopeConfig.TEXT)
+    decoded = Package.from_str(encoded)
+    assert decoded == package