python ワーニング FutureWarning: The frame.append method is deprecated and will be removed from pandas in a future version. Use pandas.concat instead.

直訳

今後の警告: dataframe.append メソッドは非推奨、将来のバージョンで pandas から削除する予定だから代わりに pandas.concat を使用してね

Ver1.4以降非推奨となっている
dataframeを結合（追加）するときに利用するメソッドで以下の様なサンプルでワーニングが出る

import pandas as pd
import numpy as np

# dataframeのカラム設定用リスト定義
COL_LIST = ['no','name','level','type1','type2','helditem']

# 結合用のdataframe作成
df1 = pd.DataFrame([[25,'pika',88,'Electric',np.NaN,np.NaN],
                   [131,'rara',80,'Water','Ice',np.NaN],
                   [143,'gon',82,'Normal',np.NaN,np.NaN]],
                  columns=COL_LIST)

# 結合用のdataframe作成
df2 = pd.DataFrame([[3,'fushi',84,'Grass','Poison',np.NaN],
                   [6,'kage',84,'Fire','Flying',np.NaN],
                   [9,'kame',84,'Water',np.NaN,np.NaN]],
                  columns=COL_LIST)

df1 = df1.append(df2)

# df1を追加したdf確認
print(df1)
"""
    no   name  level     type1   type2  helditem
0   25   pika     88  Electric     NaN       NaN
1  131   rara     80     Water     Ice       NaN
2  143    gon     82    Normal     NaN       NaN
3    3  fushi     84     Grass  Poison       NaN
4    6   kage     84      Fire  Flying       NaN
5    9   kame     84     Water     NaN       NaN
"""

対処法

import pandas as pd
import numpy as np

# dataframeのカラム設定用リスト定義
COL_LIST = ['no','name','level','type1','type2','helditem']

# 結合用のdataframe作成
df1 = pd.DataFrame([[25,'pika',88,'Electric',np.NaN,np.NaN],
                   [131,'rara',80,'Water','Ice',np.NaN],
                   [143,'gon',82,'Normal',np.NaN,np.NaN]],
                  columns=COL_LIST)

# 結合用のdataframe作成
df2 = pd.DataFrame([[3,'fushi',84,'Grass','Poison',np.NaN],
                   [6,'kage',84,'Fire','Flying',np.NaN],
                   [9,'kame',84,'Water',np.NaN,np.NaN]],
                  columns=COL_LIST)

# df1 = df1.append(df2)
df1 = pd.concat([df1,df2])    # ←こっちに変更

# df1を追加したdf確認
print(df1)
"""
    no   name  level     type1   type2  helditem
0   25   pika     88  Electric     NaN       NaN
1  131   rara     80     Water     Ice       NaN
2  143    gon     82    Normal     NaN       NaN
3    3  fushi     84     Grass  Poison       NaN
4    6   kage     84      Fire  Flying       NaN
5    9   kame     84     Water     NaN       NaN
"""