Add pytest-specific linters

Signed-off-by: Andrej Shadura <andrew.shadura@collabora.co.uk>

setup.cfg

@@ -45,6 +45,7 @@ lint =
     flake8-blind-except
     flake8-builtins
     flake8-isort
+    flake8-pytest-style
 
 [options.data_files]
 share/doc/obs-proxy/examples = proxy.conf
@@ -98,6 +99,8 @@ ignore =
 # - D104: Missing docstring in public package
 # - D105: Missing docstring in magic method
 # - D107: Missing docstring in __init__
+pytest-fixture-no-parentheses = yes
+pytest-mark-no-parentheses = yes
 
 [isort]
 # from X import (