The dependencies in Alphas are currently >= 2.0.0 and should be >=2.0.0a0 in order to work with Alphas in cases which are not PEP 440-compliant.

According to https://www.python.org/dev/peps/pep-0440/, >= 2.0.0 should also work with alpha/beta releases (a1/a2) but in some cases it does not (https://apache-airflow.slack.com/archives/C0146STM600/p1602774750041800) 

Changing to ">=2.0.0a0" should help.
provider_packages/setup_provider_packages.py
"""Setup.py for the Provider packages of Airflow project."""
import collections
import importlib
import json
import logging
import os
import re
import subprocess
import sys
import tempfile
import textwrap
from datetime import datetime, timedelta
from enum import Enum
from os import listdir
from os.path import dirname
from shutil import copyfile
from typing import Any, Dict, Iterable, List, NamedTuple, Optional, Set, Tuple, Type
import semver
from setuptools import Command, find_packages, setup as setuptools_setup
MY_DIR_PATH = os.path.dirname(__file__)
SOURCE_DIR_PATH = os.path.abspath(os.path.join(MY_DIR_PATH, os.pardir))
AIRFLOW_PATH = os.path.join(SOURCE_DIR_PATH, "airflow")
PROVIDERS_PATH = os.path.join(AIRFLOW_PATH, "providers")
sys.path.insert(0, SOURCE_DIR_PATH)
import tests.deprecated_classes  
from provider_packages.import_all_provider_classes import import_all_provider_classes  
from setup import PROVIDERS_REQUIREMENTS  
logger = logging.getLogger(__name__)  
PY3 = sys.version_info[0] == 3
class EntityType(Enum):