Migrate from Travis CI to Gitlab CI

[desilinguist]

This PR closes #651.

• Delete .travis.yml.
• Add .gitlab-ci.yml for Gitlab CI.
• Since Gitlab CI supports docker images natively, we use the latest miniconda3 image instead of downloading it every time. This makes things much simpler and faster.
• Update README.rst
  • Replace Travis CI badge with Gitlab CI badge.
  • Fix alt title for Azure Pipelines badge.
  • Update listed Python versions that we forgot to do with v2.5 release.

Note: Gitlab CI runs the builds for both the branch as well as the pull request but only shows a single status check on this page. That check only updates as passing if both builds pass. To see both builds, you have to click "Details" and then click on "Pipelines" at the top to see all of the pipeline builds or check the Slack channel.

[codecov[bot]]

Codecov Report

Merging #676 (9ee758b) into main (d590ece) will increase coverage by 0.03%. The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #676      +/-   ##
==========================================
+ Coverage   96.82%   96.85%   +0.03%     
==========================================
  Files          63       63              
  Lines        9098     9098              
==========================================
+ Hits         8809     8812       +3     
+ Misses        289      286       -3     

Impacted Files	Coverage Δ
tests/other/custom_metrics.py	94.11% <0.00%> (+17.64%)	:arrow_up:

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