require Python 3.7

Merge request !370 was supposed to make Python 3.7 minimal required version,
but in some places 3.6 sneaked from attention. This commit fixes that.
27 jobs for master in 50 minutes and 45 seconds (queued for 47 seconds)
Status Name Job ID Coverage
  Build Env
manual build-env:conda #204844
allowed to fail manual
manual build-env:debian #204841
allowed to fail manual
manual build-env:default #204846
allowed to fail manual
manual build-env:ubuntu #204838
allowed to fail manual
 
  Build
passed build:bleeding-edge #204859

00:14:22

passed build:debian #204850

00:02:42

passed build:latest #204857

00:02:00

passed build:no-extras #204855

00:02:26

passed build:stable #204853

00:02:33

passed build:ubuntu #204848

00:03:18

 
  Test
passed build PDF documentation #204880

00:05:14

passed build documentation #204879

00:06:04

passed check for broken links in doc #204881

00:06:54

passed check for dependencies installed #204863

00:00:27

passed check whitespace style #204861

00:00:26

passed coverage:latest #204877

00:30:30

failed test:bleeding-edge #204875
allowed to fail

00:14:36

passed test:debian #204871

00:01:35

passed test:latest #204873

00:01:46

passed test:no-extras #204867

00:01:14

passed test:stable #204865

00:02:37

passed test:ubuntu #204869

00:01:54

 
  Deploy
manual remove_coverage #204883
allowed to fail manual
manual remove_docs #204885
allowed to fail manual
passed upload coverage #204882

00:00:18

passed upload dev version docs #204886

00:00:29

passed upload documentation to the test server #204884

00:00:29

 
Name Stage Failure
failed
test:bleeding-edge Test
1.65s call     kwant/physics/tests/test_gauge.py::test_minimal_cycle_basis[2-honeycomb-half-ring]
1.56s call kwant/tests/test_kpm.py::test_check_convergence_decreasing_values
1.45s call kwant/tests/test_rmt.py::test_gaussian_distributions
============= 3 failed, 547 passed, 5 warnings in 78.57s (0:01:18) =============
Authenticating with credentials from /root/.docker/config.json
Uploading artifacts...
tests.xml: found 1 matching files

Uploading artifacts to coordinator... ok
id=204875 responseStatus=201 Created token=jdMrsj4v
ERROR: Job failed: exit code 1