[cpesuggest] Add CPE security tag for python-rsa.

I have manually checked the CPE tag is correct by checking the vendor
and product name match on the NVD site (version # does not need to
match).
https://nvd.nist.gov/products/cpe/search/results?namingFormat=2.2&keyword=cpe:/a:python-rsa_project:python-rsa&status=FINAL%2CDEPRECATED

Adding CPE security tags to a METADATA file is necessary to
enable automated vulnerability monitoring.
Bug: 396670837

Change-Id: I03ff2b1ff7ba5048c2f052c2fe68c7f9ebc64b4b

METADATA

diff --git a/METADATA b/METADATA
index 69d3a33..8117e22 100644
--- a/METADATA
+++ b/METADATA
@@ -11,6 +11,9 @@
   }
   version: "version-4.7"
   license_type: NOTICE
+  security {
+    tag: "NVD-CPE2.3:cpe:/a:python-rsa_project:python-rsa:4.7"
+  }
   last_upgrade_date {
     year: 2021
     month: 1