Update min python version from 3.6 to 3.8

The reason for picking 3.8 here is that it provides all the features
we currently have need of, and it available in the places we care about:

- debian/stable (bookworm): 3.11
- ubuntu/LTS (focal): 3.8
- emsdk: 3.9.2

Replaces: #23378
Fixes: #23387

ChangeLog.md

@@ -23,6 +23,8 @@ See docs/process.md for more on how version tagging works.
 - The minimum version of node required to run emscripten was bumped from v16.20
   to v18.  Version 4.0 was mistakenly shipped with a change that required v20,
   but that was reverted. (#23410)
+- The version of python required to run emscripten was bumped from 3.6 to 3.8.
+  (#23417)
 
 4.0.0 - 01/14/25
 ----------------

tools/building.py

@@ -476,17 +476,6 @@ def check_closure_compiler(cmd, args, env, allowed_to_fail):
   return True
 
 
-# Remove this once we require python3.7 and can use std.isascii.
-# See: https://docs.python.org/3/library/stdtypes.html#str.isascii
-def isascii(s):
-  try:
-    s.encode('ascii')
-  except UnicodeEncodeError:
-    return False
-  else:
-    return True
-
-
 def get_closure_compiler_and_env(user_args):
   env = shared.env_with_node_in_path()
   closure_cmd = get_closure_compiler()
@@ -623,7 +612,7 @@ def run_closure_cmd(cmd, filename, env):
   tempfiles = shared.get_temp_files()
 
   def move_to_safe_7bit_ascii_filename(filename):
-    if isascii(filename):
+    if filename.isascii():
       return os.path.abspath(filename)
     safe_filename = tempfiles.get('.js').name  # Safe 7-bit filename
     shutil.copyfile(filename, safe_filename)

tools/shared.py

@@ -20,9 +20,9 @@
 import sys
 import tempfile
 
-# We depend on python 3.6 for fstring support
-if sys.version_info < (3, 6):
-  print('error: emscripten requires python 3.6 or above', file=sys.stderr)
+# We depend on python 3.8 features
+if sys.version_info < (3, 8):
+  print(f'error: emscripten requires python 3.8 or above ({sys.executable} {sys.version})', file=sys.stderr)
   sys.exit(1)
 
 from . import colored_logger

tools/system_libs.py

@@ -2442,17 +2442,8 @@ def calculate(args):
   return ret
 
 
-# Once we require python 3.8 we can use shutil.copytree with
-# dirs_exist_ok=True and remove this function.
 def copytree_exist_ok(src, dst):
-  os.makedirs(dst, exist_ok=True)
-  for entry in os.scandir(src):
-    srcname = os.path.join(src, entry.name)
-    dstname = os.path.join(dst, entry.name)
-    if entry.is_dir():
-      copytree_exist_ok(srcname, dstname)
-    else:
-      shared.safe_copy(srcname, dstname)
+  shutil.copytree(src, dst, dirs_exist_ok=True)
 
 
 def install_system_headers(stamp):