trace: use temporary connections to the database

When used within multiple threads, the previous code would raise:

ProgrammingError: SQLite objects created in a thread can only be used in
that same thread. The object was created in thread id XXX and this is
thread id YYY.

Let's find a better fix.

my2to3/trace.py

@@ -4,20 +4,22 @@ from lib2to3.pgen2 import tokenize
 from lib2to3.refactor import get_fixers_from_package, RefactoringTool
 
 database = "traces.db"
-conn = sqlite3.connect(database)
 
 
 tracing_functions = []
 
 
 def create_table(table, *columns):
+  conn = sqlite3.connect(database)
   with conn:
     v = ', '.join(columns)
     conn.execute(
       "CREATE TABLE IF NOT EXISTS %s (%s, UNIQUE (%s))" % (table, v, v)
     )
+  conn.close()
 
   def insert_unique(*values):
+    conn = sqlite3.connect(database)
     with conn:
       try:
         conn.execute(
@@ -27,6 +29,7 @@ def create_table(table, *columns):
       except sqlite3.IntegrityError as e:
         if not str(e).startswith('UNIQUE constraint failed:'):
           raise
+    conn.close()
   return insert_unique
 
 
@@ -43,8 +46,10 @@ def get_data(table, columns_to_select='*', conditions={}):
   )
   if conditions:
     query += " WHERE " + ' AND '.join(k + " = :" + k for k in conditions.keys())
+  conn = sqlite3.connect(database)
   with conn:
     return conn.execute(query, conditions).fetchall()
+  conn.close()
 
 
 def apply_fixers(string, name):