Add toplevel metadata.

[grahamgower]

Closes #275.

[codecov]

Codecov Report

Merging #392 (d89f441) into main (4d3c3f2) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #392   +/-   ##
=======================================
  Coverage   99.87%   99.87%           
=======================================
  Files           5        5           
  Lines        1545     1552    +7     
=======================================
+ Hits         1543     1550    +7     
  Misses          2        2           

Impacted Files	Coverage Δ
demes/demes.py	100.00% <100.00%> (ø)
demes/load_dump.py	100.00% <0.00%> (ø)

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[jeromekelleher]

LGTM, one suggestion