Fix venv script shebangs. (#2122)

Previously venv script shebangs were derived from the interpreter used
to create the venv and not the resulting interpreter created in the
venv. In some scenarios this could lead to a shebang that did not
match any Python binary path available in the venv and lead to errors.

Fixes #2119

pex/interpreter.py

@@ -535,7 +535,6 @@ def _find_pyvenv_cfg(cls, maybe_venv_python_binary):
     @classmethod
     def _resolve_pyvenv_canonical_python_binary(
         cls,
-        real_binary,  # type: str
         maybe_venv_python_binary,  # type: str
     ):
         # type: (...) -> Optional[str]
@@ -581,16 +580,11 @@ def canonicalize_path(cls, path):
         N.B.: If the path is a venv symlink it will not be fully de-referenced in order to maintain
         fidelity with the requested venv Python binary choice.
         """
-        real_binary = os.path.realpath(path)
-
-        # If the path is a PEP-405 venv interpreter symlink we do not want to resolve outside of the
+        # If the path is a PEP-405 venv interpreter symlink we do not want to resolve outside the
         # venv in order to stay faithful to the binary path choice.
-        return (
-            cls._resolve_pyvenv_canonical_python_binary(
-                real_binary=real_binary, maybe_venv_python_binary=path
-            )
-            or real_binary
-        )
+        return cls._resolve_pyvenv_canonical_python_binary(
+            maybe_venv_python_binary=path
+        ) or os.path.realpath(path)
 
     class Error(Exception):
         pass

pex/pex_bootstrapper.py

@@ -547,7 +547,10 @@ def ensure_venv(
                         pex,
                         bin_path=pex_info.venv_bin_path,
                         python=os.path.join(
-                            short_venv_dir, "venv", "bin", os.path.basename(pex.interpreter.binary)
+                            short_venv_dir,
+                            "venv",
+                            "bin",
+                            os.path.basename(virtualenv.interpreter.binary),
                         ),
                         collisions_ok=collisions_ok,
                         symlink=symlink,