Backslash is swallowed when unescaping sequence like `\E\R\`

[fathom-piotr]

See this example piece of code:

msg = hl7.Message(separator="\r", sequence=[["MSH", "^~\\&"]])
unescaped = msg.unescape("\\E\\R\\")

unescaped should be \R\ because:

• the first \E\ becomes a literal \
• we're left with R\

The library, however, returns \R, as if the trailing slash could be skipped.