create EventsCalendarSpider subclass

[rhetr]

PR for #16.

from the class comment:

    Subclass of CityScrapersSpider that may be useful
    for WordPress sites with the Events Calendar plugin.
    Three additional things need to be implemented when subclassing:
        1. a categories dict
        2. _parse_location(self, item):
        3. _parse_links(self, item)
    where `item` is the dict retrieved from the API.

    categories dict should be of the following format:
    categories = {
        CLASSIFICATION: ["category-1", "category-2"],
        ..
    }

example usage:

from city_scrapers_core.spiders import EventsCalendarSpider

class AtlBeltlineSpider(EventsCalendarSpider):
    name = "atl_beltline"
    state = "GA"
    agency = "Atlanta BeltLine Inc."
    timezone = "America/New_York"
    start_urls = ["https://beltline.org/wp-json/tribe/events/v1/events"]
    categories = {
        BOARD: [
            "abi-board-meetings",
            "abi-execeutive-committee-meetings",
            "leadership",
        ],
        FORUM: [
            "community-meetings",
            "economic-development",
            "housing",
            "bahab-meetings",
        ],
        ADVISORY_COMMITTEE: ["design-review-committee"],
    }

    def _parse_location(self, item):
        ...

    def _parse_links(self, item):
        ...