<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def get_signer_by_version(digest, ver):
 """Returns a new signer object for a digest and version combination. Keyword arguments: digest -- a callable that may be passed to the initializer of any Signer object in this library. The callable must return a hasher object when called with no arguments. ver -- the version of the signature. This may be any value convertible to an int. """

    if int(ver) == 1:
        return V1Signer(digest)
    elif int(ver) == 2:
        return V2Signer(digest)
    else:
        return None