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.
19 jobs for master in 10 minutes and 1 second (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Build Env
manual build-env:conda #200191
allowed to fail manual
manual build-env:debian #200190
allowed to fail manual
manual build-env:default #200192
allowed to fail manual
manual build-env:ubuntu #200189
allowed to fail manual
 
  Build
passed build:debian #200194

00:02:39

passed build:latest #200197

00:02:01

passed build:no-extras #200196

00:02:31

passed build:stable #200195

00:02:33

passed build:ubuntu #200193

00:03:15

 
  Test
passed build PDF documentation #200206

00:06:43

passed build documentation #200205

00:06:14

failed check for broken links in doc #200207
allowed to fail

00:03:10

passed check for dependencies installed #200199

00:00:36

passed check whitespace style #200198

00:00:37

passed test:debian #200203

00:01:53

passed test:latest #200204

00:02:14

passed test:no-extras #200201

00:01:12

passed test:stable #200200

00:02:13

passed test:ubuntu #200202

00:01:59

 
Name Stage Failure
failed
check for broken links in doc Test
Traceback (most recent call last):
File "/usr/local/envs/kwant-latest/lib/python3.9/site-packages/nbclient/client.py", line 351, in _async_cleanup_kernel
assert self.km is not None
AssertionError
make: *** [linkcheck] Error 2
Makefile:39: recipe for target 'linkcheck' failed
make: Leaving directory '/builds/pablopiskunow/kwant/doc'
Authenticating with credentials from /root/.docker/config.json
ERROR: Job failed: exit code 1