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 all providers for regular packages.
    :return: list of providers that are considered for provider packages
    """
    return [prov for prov in PROVIDERS_REQUIREMENTS.keys()]
if __name__ == "__main__":
    LIST_PROVIDERS_PACKAGES = "list-providers-packages"
    LIST_BACKPORTABLE_PACKAGES = "list-backportable-packages"
    UPDATE_PACKAGE_RELEASE_NOTES = "update-package-release-notes"
    BACKPORT_PACKAGES = (os.getenv('BACKPORT_PACKAGES') == "true")
    suffix = ""
    possible_first_params = get_provider_packages()
    possible_first_params.append(LIST_PROVIDERS_PACKAGES)
    possible_first_params.append(LIST_BACKPORTABLE_PACKAGES)
    possible_first_params.append(UPDATE_PACKAGE_RELEASE_NOTES)
    if len(sys.argv) == 1:
        print("""
ERROR! Missing first param"
""", file=sys.stderr)
        usage()
        sys.exit(1)
    if sys.argv[1] == "--version-suffix":
        if len(sys.argv) < 3: