man: Remove FileType argument types in parser standard options arguments

[echoix]

There was another ResourceWarning error in manual generation that I solved after #5560. The unclosed files were defined in the argparse arguments with FileType type.

The argparse FileType argument type will be deprecated in Python 3.14. It does not properly close files if a subsequent error in another FileType argument occurs when parsing. The suggestion is to use context managers after parsing the arguments. The implementation of the changes is inspired by a PR doing the same in the CPython repo.

Other small changes included in (wasn't worth a separate PR):

• Removing urlopen()'s proxies argument. It was for the old interface. Default proxy handling is used now. It allowed correct type checking.
• Invalid None assignment to nothing in res.append(line) if res is not None else None, replaced with elif res is not None: and res.append(line), as intended 10+ years ago.
• Initialized optname variable in function split_in_groups if ever the first condition wasn't executed for a previous line (for type checking, as it was unbounded otherwise)
• Renamed variable of argument destination with typo from htmlparmas to htmlparams (two occurrences only, used once)
• Added a note to an append operation where type checking clearly indicates an error, but changes would be out of scope here.
• Collapsed a if line.startswith("/*"): continue that was not required because of the condition above, that already checked for this situation just above.

Type checking was added to touched/impacted functions to validate correct usages.