Scriv produces an obscure error when encountering an invalid Version during collection

Description

With scriv==1.3.1, running collection on a changelog that includes a heading that does not contain a valid Version (as defined by scriv.util) produces a fairly obscure error.

$ scriv collect --keep --version 1.1
Collecting from changelog.d
Reading changelog CHANGELOG.md
Traceback (most recent call last):
  File "/home/snoopjedi/repos/scriv-src/.direnv/python-3.7.16/bin/scriv", line 8, in <module>
    sys.exit(cli())
  File "/home/snoopjedi/repos/scriv-src/.direnv/python-3.7.16/lib/python3.7/site-packages/click/core.py", line 1157, in __call__
    return self.main(*args, **kwargs)
  File "/home/snoopjedi/repos/scriv-src/.direnv/python-3.7.16/lib/python3.7/site-packages/click/core.py", line 1078, in main
    rv = self.invoke(ctx)
  File "/home/snoopjedi/repos/scriv-src/.direnv/python-3.7.16/lib/python3.7/site-packages/click/core.py", line 1688, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/home/snoopjedi/repos/scriv-src/.direnv/python-3.7.16/lib/python3.7/site-packages/click/core.py", line 1434, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/home/snoopjedi/repos/scriv-src/.direnv/python-3.7.16/lib/python3.7/site-packages/click/core.py", line 783, in invoke
    return __callback(*args, **kwargs)
  File "/home/snoopjedi/repos/scriv-src/.direnv/python-3.7.16/lib/python3.7/site-packages/scriv/collect.py", line 74, in collect
    if eversion == version:
  File "/home/snoopjedi/repos/scriv-src/.direnv/python-3.7.16/lib/python3.7/site-packages/scriv/util.py", line 96, in __eq__
    return this.lstrip("v") == that.lstrip("v")
AttributeError: 'NoneType' object has no attribute 'lstrip'

The error is slightly less obscure against main (58fae87), but still confusing:

$ scriv collect --keep --version 1.1
Collecting from changelog.d
Reading changelog CHANGELOG.md
Traceback (most recent call last):
  File "/home/snoopjedi/repos/scriv-src/.direnv/python-3.7.16/bin/scriv", line 11, in <module>
    load_entry_point('scriv', 'console_scripts', 'scriv')()
  File "/home/snoopjedi/repos/scriv-src/.direnv/python-3.7.16/lib/python3.7/site-packages/click/core.py", line 1157, in __call__
    return self.main(*args, **kwargs)
  File "/home/snoopjedi/repos/scriv-src/.direnv/python-3.7.16/lib/python3.7/site-packages/click/core.py", line 1078, in main
    rv = self.invoke(ctx)
  File "/home/snoopjedi/repos/scriv-src/.direnv/python-3.7.16/lib/python3.7/site-packages/click/core.py", line 1688, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/home/snoopjedi/repos/scriv-src/.direnv/python-3.7.16/lib/python3.7/site-packages/click/core.py", line 1434, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/home/snoopjedi/repos/scriv-src/.direnv/python-3.7.16/lib/python3.7/site-packages/click/core.py", line 783, in invoke
    return __callback(*args, **kwargs)
  File "/home/snoopjedi/repos/scriv-src/src/scriv/collect.py", line 74, in collect
    if eversion == version:
  File "/home/snoopjedi/repos/scriv-src/src/scriv/util.py", line 91, in __eq__
    assert isinstance(other, Version)
AssertionError

The problem here is the Older versions heading in the CHANGELOG.md, adding  above this heading avoids the issue, but it would be nice if scriv gave a more explicit report about what went wrong in this case.

Reproduction

These files should be all that is needed to reproduce the above errors.

# pyproject.toml
[tool.scriv]
format = "md"

<!-- CHANGELOG. md -->
<a id="changelog-1.0"></a>
# 1.0

### Added

- Blah blah blah

# Older versions

<a id="changelog-0.9"></a>
# 0.9

### Added

- Blah blah blah

<!-- changelog.d/20230717_112025_snoopjedi.md -->
### Added

- Some amazing new feature

Notes

I discovered this on Python 3.7 but I believe it applies to all supported versions.

nedbat / scriv