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
release_ver = sys.argv[2]
            print()
            print()
            print(f"Preparing release version: {release_ver}")
            package_list = sys.argv[3:]
        else:
            print()
            print()
            print("Updating latest release version.")
            package_list = sys.argv[2:]
        print()
        update_release_notes_for_packages(package_list,
                                          release_version=release_ver,
                                          backport_packages=BACKPORT_PACKAGES)
        sys.exit(0)
    provider_package = sys.argv[1]
    if provider_package not in get_provider_packages():
        raise Exception(f"The package {provider_package} is not a backport package. "
                        f"Use one of {get_provider_packages()}")
    del sys.argv[1]
    print(f"Building backport package: {provider_package}")
    dependencies = PROVIDERS_REQUIREMENTS[provider_package]
    do_setup_package_providers(provider_package_id=provider_package,
                               package_dependencies=dependencies,
                               extras=find_package_extras(
                                   provider_package,
                                   backport_packages=BACKPORT_PACKAGES),
                               version_suffix=suffix,
                               backport_packages=BACKPORT_PACKAGES)