Make releasedata.Product usable in 'with' expression (#294)

This way the writing of the JSON file is handled automatically if the update does not fail.
It pave the way to further global improvements, such as a better error handling.

src/typo3.py

@@ -1,13 +1,11 @@
 from common import dates, http, releasedata
 
-product = releasedata.Product("typo3")
-data = http.fetch_url("https://get.typo3.org/api/v1/release/").json()
-for v in data:
-    if v['type'] == 'development':
-        continue
+with releasedata.ProductData("typo3") as product_data:
+    data = http.fetch_url("https://get.typo3.org/api/v1/release/").json()
+    for v in data:
+        if v['type'] == 'development':
+            continue
 
-    version = v["version"]
-    date = dates.parse_datetime(v["date"], to_utc=False)  # utc kept for now for backwards compatibility
-    product.declare_version(version, date)
-
-product.write()
+        version = v["version"]
+        date = dates.parse_datetime(v["date"], to_utc=False)  # utc kept for now for backwards compatibility
+        product_data.declare_version(version, date)