ci: Switch to new codecov uploader

[chfast]

Migrates to the new codecov uploader (old one is deprecated).
Good news is that we don't need python any more.
Instead we need to download some "binaries" (project is nodejs based).
We do some integration checks, but I'm not sure how good they are.

I also had to add CODECOV_TOKEN to Circle CI. Some docs say it shouldn't be needed for public repos on Circle CI, but upload wasn't working and worked with the token.

[codecov]

Codecov Report

Merging #615 (db58b31) into master (a621468) will increase coverage by 0.03%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #615      +/-   ##
==========================================
+ Coverage   97.18%   97.21%   +0.03%     
==========================================
  Files          74       74              
  Lines        7557     7575      +18     
==========================================
+ Hits         7344     7364      +20     
+ Misses        213      211       -2     

Flag	Coverage Δ
blockchaintests	63.61% <0.00%> (ø)
statetests	62.58% <14.70%> (-0.04%)	⬇️
unittests	94.11% <100.00%> (+0.04%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
test/state/state.hpp	96.42% <ø> (ø)
test/statetest/statetest.hpp	100.00% <ø> (ø)
test/unittests/tracing_test.cpp	100.00% <ø> (ø)
lib/evmone/baseline.cpp	100.00% <100.00%> (ø)
lib/evmone/tracing.cpp	100.00% <100.00%> (ø)
lib/evmone/tracing.hpp	94.11% <100.00%> (ø)
test/state/rlp.hpp	100.00% <100.00%> (ø)
test/state/state.cpp	96.72% <100.00%> (ø)
test/statetest/statetest_loader.cpp	89.38% <100.00%> (+0.54%)	⬆️
test/statetest/statetest_runner.cpp	100.00% <100.00%> (ø)
... and 3 more

... and 1 file with indirect coverage changes