Add the "sb" pytest fixture

• Add the "sb" pytest fixture

Usage examples:

# "sb" pytest fixture test in a method with no class
def test_sb_fixture_with_no_class(sb):
    sb.open("https://google.com/ncr")
    sb.update_text('input[title="Search"]', 'SeleniumBase\n')
    sb.click('a[href*="github.com/seleniumbase/SeleniumBase"]')
    sb.click('a[title="seleniumbase"]')


# "sb" pytest fixture test in a method inside a class
class Test_SB_Fixture():
    def test_sb_fixture_inside_class(self, sb):
        sb.open("https://google.com/ncr")
        sb.update_text('input[title="Search"]', 'SeleniumBase\n')
        sb.click('a[href*="github.com/seleniumbase/SeleniumBase"]')
        sb.click('a[title="examples"]')