rust: Make the hg-cpython crate default to Python 3

This default is used when running `cargo` manually such as for `cargo test`.
`setup.py` and `Makefile` both configure the Python major version explicitly.

Differential Revision: https://phab.mercurial-scm.org/D11618

rust/hg-cpython/Cargo.toml

@@ -9,7 +9,7 b" name='rusthg'"
 crate-type = ["cdylib"]
 
 [features]
-default = ["python27"]
+default = ["python3"]
 
 # Features to build an extension module:
 python27 = ["cpython/python27-sys", "cpython/extension-module-2-7"]

setup.py

@@ -1398,12 +1398,9 b' class RustExtension(Extension):'
 
     rusttargetdir = os.path.join('rust', 'target', 'release')
 
-    def __init__(
-        self, mpath, sources, rustlibname, subcrate, py3_features=None, **kw
-    ):
+    def __init__(self, mpath, sources, rustlibname, subcrate, **kw):
         Extension.__init__(self, mpath, sources, **kw)
         srcdir = self.rustsrcdir = os.path.join('rust', subcrate)
-        self.py3_features = py3_features
 
         # adding Rust source and control files to depends so that the extension
         # gets rebuilt if they've changed
@@ -1451,9 +1448,11 b' class RustExtension(Extension):'
 
         feature_flags = []
 
-        if sys.version_info[0] == 3 and self.py3_features is not None:
-            feature_flags.append(self.py3_features)
-            cargocmd.append('--no-default-features')
+        cargocmd.append('--no-default-features')
+        if sys.version_info[0] == 2:
+            feature_flags.append('python27')
+        elif sys.version_info[0] == 3:
+            feature_flags.append('python3')
 
         rust_features = env.get("HG_RUST_FEATURES")
         if rust_features:
@@ -1575,7 +1574,9 b' extmodules = ['
         extra_compile_args=common_cflags,
     ),
     RustStandaloneExtension(
-        'mercurial.rustext', 'hg-cpython', 'librusthg', py3_features='python3'
+        'mercurial.rustext',
+        'hg-cpython',
+        'librusthg',
     ),
 ]