Upgrade the parser to use the ExchangeList, ProductionBreakdownList,
ProductionMix, and ZoneKey classes. This should yield no functional
change. Additionally:

- Comply with Black's 88-column default line length limit.
- Define global constants to replace a number of local variables and
  literals.
- Try to use more consistent names throughout.

Refs: electricitymaps#6011, electricitymaps#6050

The JSON arrays a and b returned by the two endpoints appear to be
parallel in the sense that element a[i] corresponds with element b[i]
for all i of interest. Zip these arrays together and iterate over the
resulting pairs instead of iterating over one and performing a linear
search to find the corresponding element in the other. Additionally:

- Check whether we've been bitten by a race condition while requesting
  the above array data and bail out if so.
- Skip the first element of each array because the reported base load is
  always 0 MW.

Refs: electricitymaps#6011, electricitymaps#6050

- When a pair of requests have failed, retry them until they succeed or
  the maximum number of attempts is exceeded.
- Simplify the validation logic by making better use of the event
  classes instead of introducing extraneous data structures.

Refs: electricitymaps#6011, electricitymaps#6050

Reliably merge the source's base load array with the corresponding
electricity mix array by correlating events based on their timestamps.

Refs: electricitymaps#6011, electricitymaps#6050

- Stop discarding events whose production mix percentages exceed
  percentage thresholds, as these imposed limits may not be sound.
- Provide defaults when getting attributes from the production mix to
  avoid comparing numbers with Nones.

Refs: electricitymaps#6011, electricitymaps#6050

- New Brunswick and Newfoundland and Labrador exchange data is available
  at https://www.nspower.ca/oasis/system-reports-messages/daily-report;
  upgrade the exchange parser to leverage this new source.
- Since it no longer reduces duplication, eliminate the _get_info
  function and roll its logic into fetch_production.

Refs: electricitymaps#2541, electricitymaps#3206, electricitymaps#6011, electricitymaps#6050, electricitymaps#6317

Apply the following corrections and merge conflic resolutions:

- Fix an incorrect zone in a call to fetch_exchange.
- Treat CT generation as oil instead of gas.
- Update the comment regarding hydro capacities.
- Update the production mix thresholds for hydro and oil.

Gracefully handle errors that might occur while navigating the HTML
document containing exchange data.

Refs: electricitymaps#6011

- Re-throw caught errors to provide better diagnostic information.
- Format log messages lazily.

Refs: electricitymaps#6011

Refs: electricitymaps#6011

Add a check for 0 MW base loads to resolve the merge conflict.

Refs: electricitymaps#6011, electricitymaps#6440

Refs: electricitymaps#6011