deps: bump test PyYAML requirement to 6.0

In this patch we bump PyYAML requirement since version 6.0 has
various vulnerability fixes (see [1-3]). Since PyYAML is used only
in tests, the only reason of this patch is to remove Dependabot alerts.
After migrating to GitHub Actions issue [4] is no longer relevant,
so we are not restricted to using 5.2b1 version. There is no PyYAML 6.0
for Python 3.5 since it reached the end of its life on September 13th,
2020. So this patch removed Python 3.5 pipeline as well.

1. https://github.com/tarantool/tarantool-python/security/dependabot/1
2. https://github.com/tarantool/tarantool-python/security/dependabot/2
3. https://github.com/tarantool/tarantool-python/security/dependabot/3
4. 4f79627

Author: DifferentialOrange

.github/workflows/testing.yml

@@ -27,7 +27,6 @@ jobs:
           - '2.8'
           - '2.x-latest'
         python:
-          - '3.5'
           - '3.6'
           - '3.7'
           - '3.8'
@@ -123,7 +122,7 @@ jobs:
           - '1.10.11-0-gf0b0e7ecf-r470'
           - '2.8.3-21-g7d35cd2be-r470'
           - '2.10.0-1-gfa775b383-r486-linux-x86_64'
-        python: ['3.5', '3.6', '3.7', '3.8', '3.9', '3.10']
+        python: ['3.6', '3.7', '3.8', '3.9', '3.10']
         msgpack-deps:
           # latest msgpack will be installed as a part of requirements.txt
           - ''

requirements-test.txt

@@ -1,4 +1,2 @@
 dbapi-compliance==1.15.0
-# Use this pyyaml version to be compatible with Python34-x64 on Windows.
-# See for more details http://help.appveyor.com/discussions/problems/28023.
-pyyaml==5.2b1
+pyyaml==6.0