fixing open-ended adjustements error

While trying to prescribe an open-ended adjustments, I noticed that it currently causes an error.

When we read the start end end times of adjustments, they receive a time zone info due to their ISO format. The AWS xarray dataset does not have a time zone info (because of an [xarray limitation](pydata/xarray#3291)). So the timezone info is removed from the adjustments time bounds (l.183-184).

What was missing is that when start or end date of adjustments are blank (meaning open-start, open-ended bounds), we use a timestamp (then time-zone-naive) from the AWS dataset, and that it then causes an error later on when trying to remove the time-zone info from these same time-zone-naive bounds.

src/pypromice/qc/github_data_issues.py

@@ -177,8 +177,8 @@ def adjustData(ds,
         adj_info = adj_info.loc[adj_info.adjust_function != "time_shift", :]
 
         # if t1 is left empty, then adjustment is applied until the end of the file
-        adj_info.loc[adj_info.t0.isnull(), "t0"] = ds_out.time.values[0]
-        adj_info.loc[adj_info.t1.isnull(), "t1"] = ds_out.time.values[-1]
+        adj_info.loc[adj_info.t0.isnull(), "t0"] = pd.to_datetime(ds_out.time.values[0], utc=True)
+        adj_info.loc[adj_info.t1.isnull(), "t1"] = pd.to_datetime(ds_out.time.values[-1], utc=True)
         # making all timestamps timezone naive (compatibility with xarray)
         adj_info.t0 = pd.to_datetime(adj_info.t0).dt.tz_localize(None)
         adj_info.t1 = pd.to_datetime(adj_info.t1).dt.tz_localize(None)