Bump mypy to 0.790 for Python 3.9 compat

HashError.order is now annotated as an int to allow
HashErrors.errors.sort(key=lambda e: e.order).  Alternatively we can
define a function which assert e is not None but I prefer the more
concise version, since we never raise HashError directly anyway.

.pre-commit-config.yaml

@@ -69,7 +69,7 @@ repos:
     files: \.py$
 
 - repo: https://github.com/pre-commit/mirrors-mypy
-  rev: v0.770
+  rev: v0.790
   hooks:
   - id: mypy
     exclude: docs|tests

src/pip/_internal/exceptions.py

@@ -200,7 +200,7 @@ class HashError(InstallationError):
     """
     req = None  # type: Optional[InstallRequirement]
     head = ''
-    order = None  # type: Optional[int]
+    order = -1  # type: int
 
     def body(self):
         # type: () -> str