[Merge] master into cortex (#541)

* [MNT] Update requirements.txt (#507)

* [DOC] Fix gallery thumbnail images (#510)

* [FIX] Add check for empty stimulus (#522)

* add check for empty stimulus in stim setter for implants

* add check for empty np.ndarray

* [FIX] Fix electrode numbering annotation in implant.plot() (#523)

* fix implant annotation

* used zorder

* [MNT][FIX] Remove outdated ubuntu workflows, add Jax version requirement (#529)

* Remove 18.04, add 20.04

* Add python version to display name

* remove python version

* add jax version

* Fix jax version

* test for wheels: remove jax version

* Add jax version back

* test for wheels: remove jax version

* Add jax version back

* test for wheels

* also skip win32 for python3.7

* [FIX] Use pandas concat instead of append (#540)

---------

Co-authored-by: isaac hoffman <trsileneh@gmail.com>

pulse2percept/datasets/perezfornos2012.py

@@ -90,12 +90,12 @@ def load_perezfornos2012(shuffle=False, subjects=None, figures=None,
         for time_step in time_steps:
             time_series = np.append(time_series, row[time_step])
 
-        raw_spec = {
+        raw_spec = pd.DataFrame([{
             'figure': row['Figure'],
             'subject': row['Subject'],
             'time_series': time_series
-        }
-        time_series_df = time_series_df.append(raw_spec, verify_integrity=False,
-                                               ignore_index=True)
+        }])
+        time_series_df = pd.concat([time_series_df, raw_spec], 
+                                   verify_integrity=False, ignore_index=True)
 
     return time_series_df.reset_index(drop=True)