Upgrade to mypy 0.812 (#8748)

Fixes #8425

This PR upgrades mypy to the latest version available, 0.812.

Given the advanced type inference capabilities provided by this newer version, this PRs also fixes various type inconsistencies that are now detected. Here are the non obvious changes done to fix types:
* typing in mixins has been solved using `Protocol` classes, as recommended by mypy (https://mypy.readthedocs.io/en/latest/more_types.html#mixin-classes, https://mypy.readthedocs.io/en/stable/protocols.html)
* `cast` when we are playing with `Union` types

This PR also disables the strict optional checks that have been enable by default in recent versions of mypy. Once this PR is merged, I will create an issue to study how these checks can be enabled.

`typing.Protocol` is available only since Python 3.8. To keep compatibility with Python 3.6, I try to import the class `Protocol` from `typing`, and fallback to assign `object` to `Protocol` if that fails. This way the code is working with all versions of Python, but the mypy check can be run only with Python 3.8+ because it needs the protocol feature. As a consequence, tox runs mypy under Python 3.8.

Alternatives are:
* importing `typing_extensions`, that proposes backport of newest typing features to Python 3.6, but this implies to add a dependency to Certbot just to run mypy
* redesign the concerned classes to not use mixins, or use them differently, but this implies to modify the code itself even if there is nothing wrong with it and it is just a matter of instructing mypy to understand in which context the mixins can be used
* ignoring type for these classes with `# type: ignore` but we loose the benefit of mypy for them

* Upgrade mypy

* First step for acme

* Cast for the rescue

* Fixing types for certbot

* Fix typing for certbot-nginx

* Finalize type fixes, configure no optional strict check for mypy in tox

* Align requirements

* Isort

* Pylint

* Protocol for python 3.6

* Use Python 3.9 for mypy, make code compatible with Python 3.8<

* Pylint and mypy

* Pragma no cover

* Pythonic NotImplemented constant

* More type definitions

* Add comments

* Simplify typing logic

* Use vararg tuple

* Relax constraints on mypy

* Add more type

* Do not silence error if target is not defined

* Conditionally import Protocol for type checking only

* Clean up imports

* Add comments

* Align python version linting with mypy and coverage

* Just ignore types in an unused module

* Add comments

* Fix lint

1 files changed, 2 insertions, 1 deletions

diff --git a/tox.ini b/tox.ini
index 9f63b897c..18d0d7db3 100644
--- a/tox.ini
+++ b/tox.ini
@@ -151,11 +151,12 @@ commands =
     {[base]install_packages}
     python -m pylint --reports=n --rcfile=.pylintrc {[base]source_paths}
 
+// TODO: Re-enable strict checks for optionals with appropriate type corrections or code redesign.
 [testenv:mypy]
 basepython = python3
 commands =
     {[base]install_packages}
-    mypy {[base]source_paths}
+    mypy --no-strict-optional {[base]source_paths}
 
 [testenv:apacheconftest]
 commands =