deps: minor ICU fixes: maint docs/tool, downloader

- Docs used the word "copy" when it really meant a tool is needed.
- README-FULL-ICU.txt was generated in binary mode, but it's a
text file. This breaks on Python3 for maintaining ICU
- The ICU downloader was broken (also probably python3). It's
basically dead code since 1a25e90
landed (full icu in repo), unless someone deleted the deps/icu-small
directory from their repo.

Co-Authored-By: Christian Clauss <cclauss@me.com>
PR-URL: #32347

Reviewed-By: Ujjwal Sharma <ryzokuken@disroot.org>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Christian Clauss <cclauss@me.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

doc/guides/maintaining-icu.md

@@ -97,7 +97,7 @@ new Intl.DateTimeFormat('es', { month: 'long' }).format(new Date(9E8));
 
 …Should return `enero` not `January`.
 
-* Now, copy `deps/icu` over to `deps/icu-small`
+* Now, run the shrink tool to update `deps/icu-small` from `deps/icu`
 
 > :warning: Do not modify any source code in `deps/icu-small` !
 > See section below about floating patches to ICU.

tools/configure.d/nodedownload.py

@@ -63,7 +63,7 @@ def checkHash(targetfile, hashAlgo):
     digest = hashlib.new(hashAlgo)
     with open(targetfile, 'rb') as f:
       chunk = f.read(1024)
-      while chunk !=  "":
+      while len(chunk) > 0:
         digest.update(chunk)
         chunk = f.read(1024)
     return digest.hexdigest()

tools/icu/shrink-icu-src.py

@@ -128,14 +128,12 @@ def icu_info(icu_full_path):
 readme_name = os.path.join(options.icudst, "README-FULL-ICU.txt" )
 
 # Now, print a short notice
-fi = open(readme_name, 'wb')
-print("ICU sources - auto generated by shrink-icu-src.py", file=fi)
-print("", file=fi)
-print("This directory contains the ICU subset used by --with-intl=full-icu", file=fi)
-print("It is a strict subset of ICU %s source files with the following exception(s):" % (icu_ver_major), file=fi)
-print("* %s : compressed data file" % (dst_cmp_datafile), file=fi)
-print("", file=fi)
-print("", file=fi)
-print("To rebuild this directory, see ../../tools/icu/README.md", file=fi)
-print("", file=fi)
-fi.close()
+msg_fmt = """\
+ICU sources - auto generated by shrink-icu-src.py\n
+This directory contains the ICU subset used by --with-intl=full-icu
+It is a strict subset of ICU {} source files with the following exception(s):
+* {} : compressed data file\n\n
+To rebuild this directory, see ../../tools/icu/README.md\n"""
+
+with open(readme_name, 'w') as out_file:
+    print(msg_fmt.format(icu_ver_major, dst_cmp_datafile), file=out_file)