set matplotlib backend to Agg

to avoid useless complaints about missing TkInter


Former-commit-id: 28cc2814cdecc8624a779b4cca72f43d62f6ed71

ippisite/ippidb/management/commands/pca.py

@@ -5,8 +5,9 @@ import base64
 
 from django.core.management import BaseCommand, CommandError
 from django.forms.models import model_to_dict 
-import pandas as pd
 import matplotlib.pyplot as plt
+matplotlib.use('Agg')
+import pandas as pd
 import numpy as np
 from sklearn.decomposition import PCA
 from sklearn.preprocessing import StandardScaler