Replaced unnecessary iadd operations on lists in tokenization_utils.py with proper list methods
# Replaced unnecessary iadd operations on lists in tokenization_utils.py with proper list methods

@mfuntowicz

Previously, unnecessarily many list objects are created because of list updates through iadd operations.
This is bad for the following reasons.
* It slows down the program.
* It's a substandard style.

Regarding the slowing down, please see the following snippets.
```
l = []
for i in range(10**6):
    l.append(i)
# Takes 0.13282 seconds, on average, on my machine
```

```
l = []
for i in range(10**6):
    l += [i]  # this creates a new list [i] every iteration
# Takes 0.14698 seconds, on average, on my machine
```

The previous style is considered bad since it's confusing. It is easy to think that `l += [i]` has the same semantics as `l = l + [i]`, which is not at all the case. To see this, run the following code.
```
 l = []
for i in range(10**6):
    l = l + [i]  # This replaces the existing list with a new list (l + [i]) every iteration
```

The fact that the existing list is mutated is more clearly expressed in the new code, and, to my best knowledge, all the python standard library code prefer the style of the new code.

Thanks for the quick reviews both of you!

@LysandreJik 
I just searched the whole code and I found many more list append operations than updates through `__iadd__`. So I actually think that for consistency over the whole project scope, it may be better to use list append whenever it's applicable. If you approve, I would like to fix the other similar cases, too.

Regarding tuples, the current style seems perfectly okay. Tuples are immutable and `__iadd__` does not update existing tuple but replace it with a newly created one, i.e., `tup += ('a',)` is equivalent to `tup = tup + ('a',)`. So they should be treated differently from lists.

Here is a demonstration that shows the different effects of `__iadd__` on lists and tuples.
```
l = ['a']
print(id(l))  # 4327718256
l += ['b']
print(id(l))  # 4327718256


tup = ('a',)
print(id(tup)) # 4326480784
tup += ('b',)
print(id(l))   # 4327718256  (ID changed!)
```