Improved consistency of unreachable code. Previously, unreachable cod…

[erictraut]

…e was not supported for if or else suites when the condition type was narrowed to Never. This addresses microsoft/pylance-release#6028.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

pandera (https://github.com/pandera-dev/pandera)
-   /tmp/mypy_primer/projects/pandera/tests/core/test_schema_components.py:475:52 - error: Cannot access attribute "tolist" for class "Iterable[Unknown]"
-     Attribute "tolist" is unknown (reportAttributeAccessIssue)
- 2032 errors, 25 warnings, 0 informations 
+ 2031 errors, 25 warnings, 0 informations 

dd-trace-py (https://github.com/DataDog/dd-trace-py)
+ /tmp/mypy_primer/projects/dd-trace-py/ddtrace/appsec/_remoteconfiguration.py
+   /tmp/mypy_primer/projects/dd-trace-py/ddtrace/appsec/_remoteconfiguration.py:206:41 - error: "rc_asm_enabled" is possibly unbound (reportPossiblyUnboundVariable)
+   /tmp/mypy_primer/projects/dd-trace-py/ddtrace/appsec/_remoteconfiguration.py:207:12 - error: "rc_asm_enabled" is possibly unbound (reportPossiblyUnboundVariable)
+   /tmp/mypy_primer/projects/dd-trace-py/ddtrace/appsec/_remoteconfiguration.py:212:17 - error: "rc_asm_enabled" is possibly unbound (reportPossiblyUnboundVariable)
+   /tmp/mypy_primer/projects/dd-trace-py/ddtrace/appsec/_remoteconfiguration.py:215:16 - error: "rc_asm_enabled" is possibly unbound (reportPossiblyUnboundVariable)
-   /tmp/mypy_primer/projects/dd-trace-py/ddtrace/appsec/_iast/taint_sinks/_base.py:132:28 - error: Expected no arguments to "Evidence" constructor (reportCallIssue)
+   /tmp/mypy_primer/projects/dd-trace-py/ddtrace/appsec/_iast/taint_sinks/_base.py:134:72 - error: "evidence" is possibly unbound (reportPossiblyUnboundVariable)
-   /tmp/mypy_primer/projects/dd-trace-py/tests/utils.py:1303:17 - error: Expression of type "datetime" is incompatible with declared type "int"
-     "datetime" is incompatible with "int" (reportAssignmentType)
- 11603 errors, 583 warnings, 0 informations 
+ 11606 errors, 583 warnings, 0 informations 

pydantic (https://github.com/samuelcolvin/pydantic)
+   /tmp/mypy_primer/projects/pydantic/pydantic/json_schema.py:2348:25 - error: Cannot access attribute "append" for class "Unbound" (reportAttributeAccessIssue)
- 256 errors, 46 warnings, 0 informations 
+ 257 errors, 46 warnings, 0 informations 

freqtrade (https://github.com/freqtrade/freqtrade)
+   /tmp/mypy_primer/projects/freqtrade/freqtrade/rpc/telegram.py:547:16 - error: "message" is possibly unbound (reportPossiblyUnboundVariable)
- 595 errors, 44 warnings, 0 informations 
+ 596 errors, 44 warnings, 0 informations 

trio (https://github.com/python-trio/trio)
-   /tmp/mypy_primer/projects/trio/src/trio/_core/_tests/test_run.py:2271:12 - error: Type of "coro" is unknown (reportUnknownMemberType)
-   /tmp/mypy_primer/projects/trio/src/trio/_core/_tests/test_run.py:2271:12 - error: Type of "throw" is unknown (reportUnknownMemberType)
-   /tmp/mypy_primer/projects/trio/src/trio/_core/_tests/test_run.py:2272:28 - error: Type of "Error" is unknown (reportUnknownMemberType)
-   /tmp/mypy_primer/projects/trio/src/trio/_core/_tests/test_run.py:2273:10 - error: Type of "raises" is unknown (reportUnknownMemberType)
-   /tmp/mypy_primer/projects/trio/src/trio/_subprocess_platform/waitid.py:108:9 - error: Argument type is partially unknown
-     Argument corresponds to parameter "args" in function "spawn_system_task"
-     Argument type is "*tuple[Unknown, Event]" (reportUnknownArgumentType)
-   /tmp/mypy_primer/projects/trio/src/trio/_tests/test_channel.py:384:10 - error: Type of "raises" is unknown (reportUnknownMemberType)
-   /tmp/mypy_primer/projects/trio/src/trio/_tests/test_channel.py:390:14 - error: Type of "raises" is unknown (reportUnknownMemberType)
- 3760 errors, 52 warnings, 0 informations 
+ 3753 errors, 52 warnings, 0 informations