Remove top level await xdoctest fixture (#1098)

This is now natively supported since [xdoctest
#158](https://github.com/Erotemic/xdoctest/pull/158) has been released
so no need for the monkey patching fixture anymore.

pyproject.toml

@@ -54,7 +54,7 @@ toml = "*"
 tox = "*"
 pytest-mock = "*"
 codecov = "*"
-xdoctest = "*"
+xdoctest = ">=1.2.0"
 coverage = {version = "*", extras = ["toml"]}
 pytest-timeout = "^2"
 pytest-freezer = "^0.4"