Outdated use of pandas' "append" method

Hi, thanks for your work providing this useful wrapper.

I noticed the current version is using an outdated pandas method in the wrapper "query_API" method, line 280:

df = df.append(df_pair)

I've pip installed the package as editable on my computer and changed that line for 

df = pd.concat([df, df_pair])

and it's now working.

Thanks