docs: update python contributing guide (#147)

Adds details about blacken, updates version for system tests,
and shows how to pass through pytest arguments.

Source-Author: Chris Cotter <cjcotter@google.com>
Source-Date: Mon Feb 8 17:13:36 2021 -0500
Source-Repo: googleapis/synthtool
Source-Sha: 4679e7e415221f03ff2a71e3ffad75b9ec41d87e
Source-Link: https://github.com/googleapis/synthtool/commit/4679e7e415221f03ff2a71e3ffad75b9ec41d87e
diff --git a/synth.metadata b/synth.metadata
index fec849e..cf254e7 100644
--- a/synth.metadata
+++ b/synth.metadata
@@ -4,14 +4,14 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/python-api-core.git",
-        "sha": "94c76e0873e5b2f42331d5b1ad286c1e63b61395"
+        "sha": "7273090a011c8b840d2e90e29dda3fc8f0eed792"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "33366574ffb9e11737b3547eb6f020ecae0536e8"
+        "sha": "4679e7e415221f03ff2a71e3ffad75b9ec41d87e"
       }
     }
   ],