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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _required_envs(env_vars):
 """ Parse environment variables for required values, raising a `BrowserConfig` error if they are not found. Returns a `dict` of environment variables. """

    envs = {
        key: os.environ.get(key)
        for key in env_vars
    }

    # Check for missing keys
    missing = [key for key, val in list(envs.items()) if val is None]
    if missing:
        msg = (
            u"These environment variables must be set: " + u", ".join(missing)
        )
        raise BrowserConfigError(msg)

    # Check that we support this browser
    if envs['SELENIUM_BROWSER'] not in BROWSERS:
        msg = u"Unsuppported browser: {0}".format(envs['SELENIUM_BROWSER'])
        raise BrowserConfigError(msg)

    return envs