Fix type problems found by latest mypy

Closes #73
* Move to current latest version of pytest to avoid deprecation warning
* Move to current latest version of mypy
* Correct multiple problems with type and import flagged by newer mypy

fgpyo/util/metric.py

@@ -226,7 +226,7 @@ def parse(cls, fields: List[str]) -> Any:
         return inspect.attr_from(cls=cls, kwargs=dict(zip(header, fields)), parsers=parsers)
 
     @classmethod
-    def write(cls, path: Path, *values: MetricType) -> None:
+    def write(cls, path: Path, *values: "Metric") -> None:
         """Writes zero or more metrics to the given path.
 
         The header will always be written.