Workaround pyyaml / cython incompatibility

This test was failing when it tried to install `pyyaml`:
```
rspec ./spec/dependabot/python/update_checker/pip_compile_version_resolver_spec.rb:139
```

The underlying issue is `PyYAML` isn't yet compatible with `cython` `3.0.0`.
So manually I bumped the `pyyaml` pin to `6.0.1` which includes a
holdback pin to not install the latest `cython`:
* yaml/pyyaml@c42fa3b

The better way to fix this would be to regenerate the `pip-compile`
output lockfile, but I wasn't sure I'd be able to do that since the
`imports_shared.in` file includes a recursive reference to another
requirements file... and that other requirements file doesn't appear to
exist on disk in the fixtures... from a quick glance at the test specs
code, it may dynamically generate that file. So I'd have to start it,
then drop into a debugger to pause it, then manually go look at the file
(if it's even saved to disk), and then re-run it... and even then, it
may cause other failures with the test since it updates all the other
versions...

Anyway, rather than tracking all that down, I merely tried the pin, and
low and behold it fixed the test. So I think we're good for now,
especially as `pyyaml` isn't part of the test at all, it's just a
transitive dep of the actual package under test.

python/spec/fixtures/requirements/pip_compile_imports_shared.txt

@@ -30,7 +30,7 @@ pyaml==18.11.0            # via moto
 pycparser==2.19           # via cffi
 python-dateutil==2.6.0
 pytz==2018.7              # via moto
-pyyaml==3.13              # via pyaml
+pyyaml==6.0.1             # via pyaml
 requests==2.20.1          # via aws-xray-sdk, docker, moto, responses
 responses==0.10.4         # via moto
 s3transfer==0.1.13        # via boto3