Double underscores in .rst confuse parsing, new in 3.6

[nedbat]

Whittled down from a much larger file: changes.rst is:

*  Added parameters to coverage.__init__ for options that had been set
   on the coverage object itself.

3.5 handled this correctly. 3.6 and 3.6.1 do not:

% ./pandoc3.5 -frst -tmarkdown_strict --markdown-headings=atx --wrap=none changes.rst
-   Added parameters to coverage.\_\_init\_\_ for options that had been set on the coverage object itself.

% ./pandoc3.6 -frst -tmarkdown_strict --markdown-headings=atx --wrap=none changes.rst
Error at "changes.rst_chunk" (line 2, column 1):
unexpected 'o'
expecting block

% ./pandoc3.6.1 -frst -tmarkdown_strict --markdown-headings=atx --wrap=none changes.rst
Error at "changes.rst_chunk" (line 2, column 1):
unexpected 'o'
expecting block

Fiddling with the underscores changes the behavior:

*  Added parameters to coverage.init__ for options that had been set
   on the coverage object itself.

Produces:

Error at "changes.rst_chunk" (line 4, column 1):
unexpected end of input
expecting end of input

and

*  Added parameters to coverage.init for options that had been set
   on the coverage object itself.

converts correctly.

[jgm]

Even simpler case:

a.__b__

This should just be a literal a.__b__ but pandoc fails with

Error at (line 2, column 1):
unexpected '\n'
expecting block

Here is the list of RST reader changes in 3.6:

  * RST reader:

    + Use a new one-pass parsing strategy. Instead of having an initial pass
      where we collect reference definitions, we create links with target
      `##SUBST##something` or `##REF##something` or `##NOTE##something`,
      and resolve these in a pass over the parsed AST. This allows us to handle
      link references that are not at the top level (#10281).
    + Ignore newlines in URL in explicit link (#10279).
    + Handle block level substitutions.
    + Support `:file:` on raw directive (#8584).
    + Implement option lists (#10318).
    + Avoid putting metadata in Para (#7766). Create MetaInlines when possible,
      just as with markdown input. MetaBlocks is still used when there are
      multiple paragraphs or non-paragraph content.
      This change also affects field lists.
    + Fix linked substitutions (#6588). E.g. `|Python|_`.
    + Support inline anchors (#9196).
    + Explicit links define references (#5081). For example,
      ``Go to `g`_ `g <www.example.com>`_.`` should produce two links to
      www.example.com.

[jgm]

Also:

% pandoc -f rst
a__b__
Error at (line 3, column 1):
unexpected end of input
expecting end of input

And

% pandoc -f rst
__b__
^D

(Succeeds but with no output.)