Convert all .py files to be valid Python

`raise NotImplementedError('Please replace this line with your code...')` signals to the user where they need to put their code.

This approach enables the codebase to be tested with lint tools like flake8, pyflakes, pylint, etc.

solutions/system_design/social_graph/social_graph_snippets.py

@@ -61,3 +61,4 @@ class UserGraphService(object):
     def bfs(self, source, dest):
         # Use self.visited_ids to track visited nodes
         # Use self.lookup to translate a person_id to a Person
+        pass

solutions/system_design/web_crawler/web_crawler_snippets.py

@@ -2,33 +2,33 @@
 
 class PagesDataStore(object):
 
-    def __init__(self, db);
+    def __init__(self, db):
         self.db = db
-        ...
+        pass
 
     def add_link_to_crawl(self, url):
         """Add the given link to `links_to_crawl`."""
-        ...
+        pass
 
     def remove_link_to_crawl(self, url):
         """Remove the given link from `links_to_crawl`."""
-        ...
+        pass
 
-    def reduce_priority_link_to_crawl(self, url)
+    def reduce_priority_link_to_crawl(self, url):
         """Reduce the priority of a link in `links_to_crawl` to avoid cycles."""
-        ...
+        pass
 
     def extract_max_priority_page(self):
         """Return the highest priority link in `links_to_crawl`."""
-        ...
+        pass
 
     def insert_crawled_link(self, url, signature):
         """Add the given link to `crawled_links`."""
-        ...
+        pass
 
     def crawled_similar(self, signature):
         """Determine if we've already crawled a page matching the given signature"""
-        ...
+        pass
 
 
 class Page(object):
@@ -41,7 +41,7 @@ class Page(object):
 
     def create_signature(self):
         # Create signature based on url and contents
-        ...
+        pass
 
 
 class Crawler(object):