set OS field value based on platform name instead of writing 255

adapting test_metadata to reflect the OS byte usage
adde news and ACK
python · wungad · wungad · wungad · Dec 23, 2019 · the-knights-who-say-ni
diff --git a/Misc/ACKS b/Misc/ACKS
diff --git a/Misc/NEWS.d/next/Library/2019-12-23-03-21-38.bpo-39121.HgPaAz.rst b/Misc/NEWS.d/next/Library/2019-12-23-03-21-38.bpo-39121.HgPaAz.rst
diff --git a/Lib/gzip.py b/Lib/gzip.py
diff --git a/Lib/gzip.py b/Lib/gzip.py
diff --git a/Lib/gzip.py b/Lib/gzip.py
diff --git a/Lib/gzip.py b/Lib/gzip.py
diff --git a/Lib/gzip.py b/Lib/gzip.py
@@ -17,6 +17,15 @@

 READ, WRITE = 1, 2

+# OS field values as specified in RFC 1952
+OS_CODES = {