SAI diag shell in OSS: python dep for FBOSS

Summary:
The SAI diag shell implementation requires Python3.7.6.
Previous diff in the stack added Python3.7.6 manifest.
Make use of it to build diag shell and link with Python lib.

Differential Revision: D20064315

fbshipit-source-id: c855f8a9f554066e8150ce1d867828a84161961b

build/fbcode_builder/manifests/fboss

@@ -32,6 +32,7 @@ libnl
 libsai
 OpenNSA
 re2
+Python-3.7.6
 
 [shipit.pathmap]
 fbcode/fboss/github = .