batch-file enumeration improvements (https://github.com/ytdl-org/youtube-dl/pull...

batch-file enumeration improvements (https://github.com/ytdl-org/youtube-dl/pull/26813)

Co-authored by: glenn-slayden
Modified from https://github.com/ytdl-org/youtube-dl/pull/26813/commits/c9a9ccf8a35e157e22afeaafc2851176ddd87e68

These improvements apply to reading the list of URLs from the file supplied via the `--batch-file` (`-a`) command line option.

1. Skip blank and empty lines in the file. Currently, lines with leading whitespace are only skipped when that whitespace is followed by a comment character (`#`, `;`, or `]`). This means that empty lines and lines consisting only of whitespace are returned as (trimmed) empty strings in the list of URLs to process.

2. [bug fix] Detect and remove the Unicode BOM when the file descriptor is already decoding Unicode.

With Python 3, the `batch_fd` enumerator returns the lines of the file as Unicode. For UTF-8, this means that the raw BOM bytes from the file `\xef \xbb \xbf` show up converted into a single `\ufeff` character prefixed to the first enumerated text line.

This fix solves several buggy interactions between the presence of BOM, the skipping of comments and/or blank lines, and ensuring the list of URLs is consistently trimmed. For example, if the first line of the file is blank, the BOM is incorrectly returned as a URL standing alone. If the first line contains a URL, it will be prefixed with this unwanted single character--but note that its being there will have inhibited the proper trimming of any leading whitespace. Currently, the `UnicodeBOMIE` helper attempts to recover from some of these error cases, but this fix prevents the error from happening in the first place (at least on Python3). In any case, the `UnicodeBOMIE` approach is flawed, because it is clearly illogical for a BOM to appear in the (non-batch) URL(s) specified directly on the command line (and for that matter, on URLs *after the first line* of a batch list, also)

3. Adds proper trimming of the " #" into the read_batch_urls processing so that the URLs it enumerates are cleaned and trimmed more consistently.