fixing relpath bug

[choldgraf]

This is a bug that is difficult to reproduce because it only happens when you build the documentation from a folder that is not strictly in the parent tree of the build folder. By that I mean a folder structure like this:

This works:

# Path to documentation
run/build/from/this/folder/path/to/docs
# Sphinx build command is run from here
run/build/from/this/folder

This raises an error:

# Path to documentation
run/build/from/this/folder/path/to/docs
# Sphinx build command is run from here
run/build/from/this/other/folder

The problem is in Pathlib somewhere when it is calling relative_to. This method only works if the two items share the same root, which isn't true in the second case above. So this PR just uses relpath.

[codecov]

Codecov Report

Merging #149 into master will not change coverage by %.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #149   +/-   ##
=======================================
  Coverage   83.85%   83.85%           
=======================================
  Files           9        9           
  Lines         805      805           
=======================================
  Hits          675      675           
  Misses        130      130           

Flag	Coverage Δ
#pytests	83.85% <100.00%> (ø)

Impacted Files	Coverage Δ
myst_nb/cache.py	78.44% <100.00%> (ø)

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