Deprecate the "comment" format for declaring dependencies

[pfmoore]

Now that PEP 723 support has been added, the "comment" form of declaration:

# Requirements:
#    requests

should probably be deprecated (there's little point in having three ways of doing this).

Does this need a deprecation period, a warning, or anything like that? Presumably people using the comment format can change to PEP 723 format, but we should probably give them a better transition than a silent failure.

[jaraco]

Does this need a deprecation period, a warning, or anything like that? Presumably people using the comment format can change to PEP 723 format, but we should probably give them a better transition than a silent failure.

Yes, I think so. Fortunately, the way the precedence happens, users can put both forms in their scripts to have maximal compatibility across pip-run versions, so the deprecation period can be short.