Fix pandas 2.x/3.x incompatibilities in economics/

[propcgamer20-png]

pyproject.toml declares pandas>=1.5.3 with no upper bound, so these code paths run under pandas 2.x / 3.x where the calls below no longer work.

eventstudy.py - get_all_economic_events_date_time()

for time in event_times:
    data_frame.append({'event-name': event, 'release-date-time-full': time}, ignore_index=True)
return data_frame

DataFrame.append was removed in pandas 2.0. Even on older pandas this was already broken - the result of .append() was never assigned, so the method always returned the empty frame it started with. Now collects the rows and builds the frame once:

rows = []
for event in event_names:
    for time in self.get_economic_event_date_time(event):
        rows.append({'event-name': event, 'release-date-time-full': time})
data_frame = pandas.DataFrame(rows, columns=columns)

quickchart.py

• df.fillna(method='ffill') -> df.ffill() (the method= argument is removed in pandas 3.0, deprecated since 2.1)
• datetime.datetime.utcnow() (x2) -> datetime.datetime.now(datetime.timezone.utc).replace(tzinfo=None) (utcnow() is deprecated for removal), hoisted into one now_utc. The .replace(tzinfo=None) keeps the value tz-naive, so downstream pd.Timestamp(...) behaviour is unchanged.

Tests

tests/test_eventstudy.py - the frame-builder now returns a populated frame (3 rows, correct columns) and an empty-but-correctly-shaped frame when there are no events.

$ python -m pytest tests/ -q
7 passed

Stacked on #79 so quickchart.py merges cleanly; the PR narrows once #79 lands.

Toward #72