ENH: Update XLON calendars

[gerrymanoim]

Updates XLON to include holidays from pandas_market_calendars.

Closes #163.

See details from original PR: rsheftel/pandas_market_calendars#84

[jmccorriston]

This looks right to me. However, the only material changes (I believe) are to the May Bank holiday going forward (2021 and later) and the May Bank holiday in 1995. Everything else looks to be just style changes, right?

[rsheftel]

Running the test suite gives this result:

======================================================================
ERROR: test suite for <class 'tests.test_xlon_calendar.XLONCalendarTestCase'>
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Traceback (most recent call last):
  File "/usr/local/anaconda3/envs/tc/lib/python3.8/site-packages/nose/suite.py", line 209, in run
    self.setUp()
  File "/usr/local/anaconda3/envs/tc/lib/python3.8/site-packages/nose/suite.py", line 292, in setUp
    self.setupContext(ancestor)
  File "/usr/local/anaconda3/envs/tc/lib/python3.8/site-packages/nose/suite.py", line 315, in setupContext
    try_run(context, names)
  File "/usr/local/anaconda3/envs/tc/lib/python3.8/site-packages/nose/util.py", line 471, in try_run
    return func()
  File "/home/ryan/git/trading_calendars/tests/test_trading_calendar.py", line 237, in setupClass
    cls.calendar = cls.calendar_class(cls.start_date, cls.end_date)
  File "/home/ryan/git/trading_calendars/trading_calendars/trading_calendar.py", line 131, in __init__
    _overwrite_special_dates(_all_days, self._closes, _special_closes)
  File "/home/ryan/git/trading_calendars/trading_calendars/trading_calendar.py", line 1045, in _overwrite_special_dates
    raise ValueError("Special dates %s are not trading days." % bad_dates)
ValueError: Special dates [Timestamp('1999-12-31 12:30:00+0000', tz='UTC')] are not trading days.

----------------------------------------------------------------------
Ran 0 tests in 1.751s

[gerrymanoim]

Yep - I see the same issue in the CI. Will sort it out.

[gerrymanoim]

Also yes - most of the changes were style, but I liked the styling in pandas market calendars better than what we previously had.

[gerrymanoim]

Right - the issue here is we have pd.Timestamp("1999-12-31", tz='UTC') as an adhoc holiday, but also have add special closes on NewYearsEve (normally it is a half day).

It was closed as per http://news.bbc.co.uk/2/hi/uk_news/376063.stm (eve of the 3rd millennium). Will add a fix.

[rsheftel]

This now works for the special holidays that started this PR, but it does appear that many of the old tests fail now

[gerrymanoim]

Yep - forgot to fix the CSV. Doing that now.