Fix missing pytest 4.0 features

Not really sure how many versions of pytest we should support? But then
again, we might as well for now...

pyproject.toml

@@ -51,7 +51,7 @@ Repository = "https://github.com/carpedm20/fbchat/"
 
 [tool.flit.metadata.requires-extra]
 test = [
-    "pytest~=4.0",
+    "pytest>=4.0,<6.0",
 ]
 docs = [
     "sphinx~=2.0",

pytest.ini

@@ -3,7 +3,7 @@ xfail_strict = true
 markers =
     online: Online tests, aka. disabled tests...
 addopts =
-    --strict-markers
+    --strict
     -m "not online"
 testpaths = tests
 filterwarnings = error