gh-124621: Emscripten: Support pyrepl in browser (GH-136931)

Basic support for pyrepl in Emscripten. Limitations:
* requires JSPI
* no signal handling implemented

As followup work, it would be nice to implement a webworker variant
for when JSPI is not available and proper signal handling.

Because it requires JSPI, it doesn't work in Safari. Firefox requires
setting an experimental flag. All the Chromiums have full support since
May. Until we make it work without JSPI, let's keep the original web_example
around.

Co-authored-by: Łukasz Langa <lukasz@langa.pl>
Co-authored-by: Éric <merwok@netwok.org>

Lib/test/test_fcntl.py

@@ -8,7 +8,7 @@ import struct
 import sys
 import unittest
 from test.support import (
-    cpython_only, get_pagesize, is_apple, requires_subprocess, verbose
+    cpython_only, get_pagesize, is_apple, requires_subprocess, verbose, is_emscripten
 )
 from test.support.import_helper import import_module
 from test.support.os_helper import TESTFN, unlink, make_bad_fd
@@ -211,6 +211,7 @@ class TestFcntl(unittest.TestCase):
     @unittest.skipUnless(
         hasattr(fcntl, "F_SETPIPE_SZ") and hasattr(fcntl, "F_GETPIPE_SZ"),
         "F_SETPIPE_SZ and F_GETPIPE_SZ are not available on all platforms.")
+    @unittest.skipIf(is_emscripten, "Emscripten pipefs doesn't support these")
     def test_fcntl_f_pipesize(self):
         test_pipe_r, test_pipe_w = os.pipe()
         try:
@@ -265,12 +266,14 @@ class TestFcntl(unittest.TestCase):
     @unittest.skipUnless(
         hasattr(fcntl, "F_SETOWN_EX") and hasattr(fcntl, "F_GETOWN_EX"),
         "requires F_SETOWN_EX and F_GETOWN_EX")
+    @unittest.skipIf(is_emscripten, "Emscripten doesn't actually support these")
     def test_fcntl_small_buffer(self):
         self._check_fcntl_not_mutate_len()
 
     @unittest.skipUnless(
         hasattr(fcntl, "F_SETOWN_EX") and hasattr(fcntl, "F_GETOWN_EX"),
         "requires F_SETOWN_EX and F_GETOWN_EX")
+    @unittest.skipIf(is_emscripten, "Emscripten doesn't actually support these")
     def test_fcntl_large_buffer(self):
         self._check_fcntl_not_mutate_len(2024)