acis.py: use mask instead of replace

Author: kandersolar

pvlib/iotools/acis.py

@@ -413,8 +413,8 @@ def get_acis_station_data(station, start, end, trace_val=0.001,
                  'climdiv,valid_daterange,tzo,network')
     }
     df, metadata = _get_acis(start, end, params, map_variables, url, **kwargs)
-    df = df.replace("M", np.nan)
-    df = df.replace("T", trace_val)
+    df = df.mask(df == 'M', np.nan)
+    df = df.mask(df == 'T', trace_val)
     df = df.astype(float)
     return df, metadata