make exec call compatible with Python 3.x

exec was a statement in Python 2.x [1] and it has been converted to a function
in Python 3.x [2], change introduced in version 3.0 [3].

1. https://docs.python.org/2/reference/simple_stmts.html#exec
2. https://docs.python.org/3/library/functions.html#exec
3. https://docs.python.org/3/whatsnew/3.0.html

Author: ligurio

doc/conf.py

@@ -21,7 +21,7 @@
 # Read package version without importing it
 for line in open(os.path.join(os.path.dirname(os.path.abspath('.')), "tarantool", "__init__.py")):
     if line.startswith("__version__"):
-        exec line
+        exec(line)
         break
 
 # -- General configuration -----------------------------------------------------