Enable mypy option no_implicit_reexport

This option is part of the set of "strict" mypy options, bringing the
project closer to full strict checking.

https://mypy.readthedocs.io/en/stable/command_line.html#cmdoption-mypy-no-implicit-reexport

> By default, imported values to a module are treated as exported and
> mypy allows other modules to import them. This flag changes the
> behavior to not re-export unless the item is imported using from-as or
> is included in `__all__`.

A side benefit of using `__all__` is that it allows removing the flake8
exceptions from the code base. Now `__init__.py` files are checked too.

Refs: #972

piptools/_compat/__init__.py

@@ -1,2 +1,3 @@
-# flake8: noqa
 from .pip_compat import PIP_VERSION, parse_requirements
+
+__all__ = ["PIP_VERSION", "parse_requirements"]

piptools/repositories/__init__.py

@@ -1,3 +1,4 @@
-# flake8: noqa
 from .local import LocalRequirementsRepository
 from .pypi import PyPIRepository
+
+__all__ = ["LocalRequirementsRepository", "PyPIRepository"]

setup.cfg

@@ -78,6 +78,7 @@ disallow_untyped_calls = true
 disallow_untyped_decorators = true
 ignore_missing_imports = true
 no_implicit_optional = true
+no_implicit_reexport = true
 strict_equality = true
 warn_redundant_casts = true
 warn_return_any = true