How to add months into a data frame? Python

Question

I'd like to add a column into my data frame, the final version that I'd like to have is as follows,

   Plant  Year Month
0      A  2021   1
1      B  2021   1
2      C  2021   1
3      A  2021   2
4      B  2021   2
5      C  2021   2
6      A  2021   3
7      B  2021   3
8      C  2021   3
9      A  2021   4
10     B  2021   4
11     C  2021   4

and the code is as follows,

import pandas as pd
import numpy as np

TotalMonth = [1,2,3,4]
df = pd.DataFrame()
Plants = ['A', 'B', 'C']

a = list(range(0, 4))
for m in a:
    q = range(0, len(Plants))
    for p in q:
        df = df.append({'Plant': Plants[p]}, ignore_index=True)
        df['Year'] = 2021

print(df)