Fix remaining issues with GitHub Actions

[jni]

• Docs deployment still hasn't been migrated
• MacOS Py3.9 is failing for mysterious reasons — maybe install deps from conda?
• Windows is failing in caching — also mysterious!

__________________________ test_path_length_caching ___________________________

    def test_path_length_caching():
        skeleton = Skeleton(skeleton3d)
        t0 = process_time()
        distances = skeleton.path_lengths()
        t1 = process_time()
        distances2 = skeleton.path_lengths()
        t2 = process_time()
>       assert t2 - t1 < t1 - t0
E       assert (15.84375 - 15.84375) < (15.84375 - 15.84375)

• Migrate coveralls from Travis to GHA and remove travis yml

[mkcor]

Docs deployment still hasn't been migrated

So, this means moving this functionality

skan/.travis.yml

Lines 46 to 62 in 97a217d

	- pip install doctr
	- cd doc
	- if [[ ! -f schizonts/schizont4_UninfRBC1_01.tif ]]; then
	wget -q -O s.zip https://osf.io/7vyx3/download && unzip s.zip;
	fi
	- ls -l
	- ls -l schizonts/*.tif
	- make html
	- cd ..
	- if [[ -z "$TRAVIS_TAG" ]]; then
	doctr deploy --build-tags latest;
	doctr deploy --build-tags .;
	else
	DEPLOY_DIR="version-$TRAVIS_TAG";
	doctr deploy --build-tags $DEPLOY_DIR;
	doctr deploy --build-tags stable;
	fi

into .github/workflows/build-docs.yml