All sendfile differences between Linux and Solaris were hopefully taken care
of, and hence we can reenable it within the shutil module.

This was requested upstream as well:
https://bugs.python.org/issue41843

--- Python-3.9.1/Lib/shutil.py
+++ Python-3.9.1/Lib/shutil.py
@@ -50,7 +50,7 @@ elif _WINDOWS:
     import nt
 
 COPY_BUFSIZE = 1024 * 1024 if _WINDOWS else 64 * 1024
-_USE_CP_SENDFILE = hasattr(os, "sendfile") and sys.platform.startswith("linux")
+_USE_CP_SENDFILE = hasattr(os, "sendfile")
 _HAS_FCOPYFILE = posix and hasattr(posix, "_fcopyfile")  # macOS
 
 # CMD defaults in Windows 10
--- Python-3.9.1/Doc/library/shutil.rst
+++ Python-3.9.1/Doc/library/shutil.rst
@@ -452,7 +452,8 @@ the use of userspace buffers in Python a
 
 On macOS `fcopyfile`_ is used to copy the file content (not metadata).
 
-On Linux :func:`os.sendfile` is used.
+On Linux, Solaris and other POSIX platforms where :func:`os.sendfile` supports
+copies between 2 regular file descriptors :func:`os.sendfile` is used.
 
 On Windows :func:`shutil.copyfile` uses a bigger default buffer size (1 MiB
 instead of 64 KiB) and a :func:`memoryview`-based variant of