Make tox actions work on Debian 10 (#7703)

- Remove the requirement for a specific version of Python
- Move dep comment to a separate line, Tox 3.7.0 like trailing ones

Signed-off-by: Dagfinn Ilmari Mannsåker <ilmari@ilmari.org>

changelog.d/7703.misc

@@ -0,0 +1 @@
+Make Tox actions work on Debian 10.

tox.ini

@@ -2,7 +2,6 @@
 envlist = packaging, py35, py36, py37, py38, check_codestyle, check_isort
 
 [base]
-basepython = python3.7
 deps =
     mock
     python-subunit
@@ -120,11 +119,11 @@ commands =
 
 [testenv:check_codestyle]
 skip_install = True
-basepython = python3.6
 deps =
     flake8
     flake8-comprehensions
-    black==19.10b0  # We pin so that our tests don't start failing on new releases of black.
+    # We pin so that our tests don't start failing on new releases of black.
+    black==19.10b0
 commands =
     python -m black --check --diff .
     /bin/sh -c "flake8 synapse tests scripts scripts-dev synctl {env:PEP8SUFFIX:}"
@@ -140,7 +139,6 @@ skip_install = True
 deps = towncrier>=18.6.0rc1
 commands =
    python -m towncrier.check --compare-with=origin/develop
-basepython = python3.6
 
 [testenv:check-sampleconfig]
 commands = {toxinidir}/scripts-dev/generate_sample_config --check