Remove redundant CI jobs

[jngrad]

First PR for #2694

Description of changes:

• updated MacOS X jobs and some Ubuntu 18 jobs to Python3
• removed redundant Python2 jobs: 2 in GitLab CI and 1 in Travis CI
• there should be no difference in coverage (Update containers: Scafacos, 4.0.2 docker#78)

[mkuron]

Please check https://hub.docker.com/u/espressomd. These images are used by Travis. All images except for espresso-ubuntu and espresso-ubuntu-python3 should be deleted. And these two seem to have not been automatically rebuilt in almost a year for some reason.

[jngrad]

@mkuron thanks for the link, the Dockerfile there really needs an update. The .travis.yml here too.

[codecov]

Codecov Report

Merging #2834 into python will decrease coverage by <1%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           python   #2834    +/-   ##
=======================================
- Coverage      83%     82%    -1%     
=======================================
  Files         517     514     -3     
  Lines       26895   26850    -45     
=======================================
- Hits        22374   22210   -164     
- Misses       4521    4640   +119

Impacted Files	Coverage Δ
src/core/galilei.cpp	0% <0%> (-92%)	⬇️
src/core/errorhandling.cpp	55% <0%> (-30%)	⬇️
src/core/communication.cpp	73% <0%> (-22%)	⬇️
..._interface/accumulators/AutoUpdateAccumulators.hpp	87% <0%> (-13%)	⬇️
src/core/MpiCallbacks.hpp	95% <0%> (-1%)	⬇️
src/core/integrate.cpp	73% <0%> (-1%)	⬇️
src/core/electrostatics_magnetostatics/p3m.cpp	85% <0%> (-1%)	⬇️
...ore/grid_based_algorithms/lb_particle_coupling.cpp	91% <0%> (-1%)	⬇️
src/core/particle_data.cpp	96% <0%> (-1%)	⬇️
src/core/unit_tests/MpiCallbacks_test.cpp	100% <0%> (ø)	⬆️
... and 26 more

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[jngrad]

The remaining Docker containers and CI jobs will be converted to Python 3 later (hopefully for the next ES meeting). Merging this now will reduce the load on CI for new PRs.

[fweik]

bors r+

[bors]

Build succeeded

• ICP GitLab CI