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
"""
    Returns prefix for provider CHANGES files.
    """
    if backport_packages:
        return "BACKPORT_PROVIDERS_CHANGES_"
    else:
        return "PROVIDERS_CHANGES_"
def get_all_releases(provider_package_path: str, backport_packages: bool) -> List[ReleaseInfo]:
    """
    Returns information about past releases (retrieved from BACKPORT_PROVIDERS_CHANGES_ files stored in the
    package folder.
    :param provider_package_path: path of the package
    :param backport_packages: whether to prepare regular (False) or backport (True) packages
    :return: list of releases made so far.
    """
    changes_file_prefix = get_provider_changes_prefix(backport_packages=backport_packages)
    past_releases: List[ReleaseInfo] = []
    changes_file_names = listdir(provider_package_path)
    for file_name in sorted(changes_file_names, reverse=True):
        if file_name.startswith(changes_file_prefix) and file_name.endswith(".md"):
            changes_file_path = os.path.join(provider_package_path, file_name)
            with open(changes_file_path, "rt") as changes_file:
                content = changes_file.read()
            found = re.search(r'/([a-z0-9]*)\)', content, flags=re.MULTILINE)
            if not found:
                print("No commit found. This seems to be first time you run it", file=sys.stderr)