gh-90548: Make musl test skips smarter (fixes Alpine errors) (#131313)

* Make musl test skips smarter (fixes Alpine errors)

A relatively small number of tests fail when the underlying c library is
provided by musl.  This was originally reported in bpo-46390 by
Christian Heimes.  Among other changes, these tests were marked for
skipping in gh-31947/ef1327e3 as part of bpo-40280 (emscripten support),
but the skips were conditioned on the *platform* being emscripten (or
wasi, skips for which ere added in 9b50585e02).

In gh-131071 Victor Stinner added a linked_to_musl function to enable
skipping a test in test_math that fails under musl, like it does on a
number of other platforms.  This check can successfully detect that
python is running under musl on Alpine, which was the original problem
report in bpo-46390.

This PR replaces Victor's solution with an enhancement to
platform.libc_ver that does the check more cheaply, and also gets the
version number.  The latter is important because the math test being
skipped is due to a bug in musl that has been fixed, but as of this
checkin date has not yet been released.  When it is, the test skip can
be fixed to check for the minimum needed version.

The enhanced version of linked_to_musl is also used to do the skips of
the other tests that generically fail under musl, as opposed to
emscripten or wasi only failures.  This will allow these tests to be
skipped automatically on Alpine.

This PR does *not* enhance libc_ver to support emscripten and wasi, as
I'm not familiar with those platforms; instead it returns a version
triple of (0, 0, 0) for those platforms.  This means the musl tests will
be skipped regardless of musl version, so ideally someone will add
support to libc_ver for these platforms.

* Platform tests and bug fixes.

In adding tests for the new platform code I found a bug in the old code:
if a valid version is passed for version and it is greater than the
version found for an so *and* there is no glibc version, then the
version from the argument was returned.  The code changes here fix
that.

* Add support docs, including for some preexisting is_xxx's.

* Add news item about libc_ver enhancement.

* Prettify platform re expression using re.VERBOSE.

Lib/platform.py

@@ -189,22 +189,25 @@ def libc_ver(executable=None, lib='', version='', chunksize=16384):
             # sys.executable is not set.
             return lib, version
 
-    libc_search = re.compile(b'(__libc_init)'
-                          b'|'
-                          b'(GLIBC_([0-9.]+))'
-                          b'|'
-                          br'(libc(_\w+)?\.so(?:\.(\d[0-9.]*))?)', re.ASCII)
+    libc_search = re.compile(br"""
+          (__libc_init)
+        | (GLIBC_([0-9.]+))
+        | (libc(_\w+)?\.so(?:\.(\d[0-9.]*))?)
+        | (musl-([0-9.]+))
+        """,
+        re.ASCII | re.VERBOSE)
 
     V = _comparable_version
     # We use os.path.realpath()
     # here to work around problems with Cygwin not being
     # able to open symlinks for reading
     executable = os.path.realpath(executable)
+    ver = None
     with open(executable, 'rb') as f:
         binary = f.read(chunksize)
         pos = 0
         while pos < len(binary):
-            if b'libc' in binary or b'GLIBC' in binary:
+            if b'libc' in binary or b'GLIBC' in binary or b'musl' in binary:
                 m = libc_search.search(binary, pos)
             else:
                 m = None
@@ -216,7 +219,7 @@ def libc_ver(executable=None, lib='', version='', chunksize=16384):
                     continue
                 if not m:
                     break
-            libcinit, glibc, glibcversion, so, threads, soversion = [
+            libcinit, glibc, glibcversion, so, threads, soversion, musl, muslversion = [
                 s.decode('latin1') if s is not None else s
                 for s in m.groups()]
             if libcinit and not lib:
@@ -224,18 +227,22 @@ def libc_ver(executable=None, lib='', version='', chunksize=16384):
             elif glibc:
                 if lib != 'glibc':
                     lib = 'glibc'
-                    version = glibcversion
-                elif V(glibcversion) > V(version):
-                    version = glibcversion
+                    ver = glibcversion
+                elif V(glibcversion) > V(ver):
+                    ver = glibcversion
             elif so:
                 if lib != 'glibc':
                     lib = 'libc'
-                    if soversion and (not version or V(soversion) > V(version)):
-                        version = soversion
-                    if threads and version[-len(threads):] != threads:
-                        version = version + threads
+                    if soversion and (not ver or V(soversion) > V(ver)):
+                        ver = soversion
+                    if threads and ver[-len(threads):] != threads:
+                        ver = ver + threads
+            elif musl:
+                lib = 'musl'
+                if not ver or V(muslversion) > V(ver):
+                    ver = muslversion
             pos = m.end()
-    return lib, version
+    return lib, version if ver is None else ver
 
 def _norm_version(version, build=''):