chore: bump required minimum version of coincurve

[danceratopz]

What was wrong?

ethereum/execution-spec-tests installs the ethereum package provided by execution-specs as a dependency. Both execution-spec-tests and execution-specs require coincurve and currently require >=17,<18.

We'd like to upgrade the required minimum major version of coincurve from 17 to 18 in order to get full Python 3.11 support (ethereum/execution-spec-tests#280). This simplifies installation of execution-test-specs on macOS cf ethereum/execution-spec-tests#274.

Before we can do that, we need execution-specs to upgrade first :-)

How was it fixed?

Cute Animal Picture

"Psst, plz upgrade to coincurve 18!"

[codecov-commenter]

Codecov Report

Patch and project coverage have no change.

Comparison is base (2a592a8) 74.06% compared to head (198893e) 74.06%.
Report is 10 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #827   +/-   ##
=======================================
  Coverage   74.06%   74.06%           
=======================================
  Files         571      571           
  Lines       31735    31735           
=======================================
  Hits        23503    23503           
  Misses       8232     8232           

Flag	Coverage Δ
unittests	74.06% <ø> (ø)

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[petertdavies]

LGTM