Exclude if TYPE_CHECKING: blocks from coverage

This is extracted from
#188 as a separate PR.

This commit also adds includes for per-project overriding or extending
of the coverage excludes:

* If a project has a `.cookiecutter/includes/coverage/exclude_also` file
  that will _replace_ the cookiecutter's default `exclude_also`'s

* If a project has a `.cookiecutter/includes/coverage/exclude_also/tail` file
  that will _extend_ the cookiecutter's default `exclude_also`'s

Also fix a bug in `local_extensions.py`'s `include_exists(path)` function:
don't crash if there's a _folder_ (rather than a file) at `path` (the
code was catching the wrong exception class, looks like just a mistake).

_shared/local_extensions.py

@@ -150,7 +150,7 @@ def include_exists(self, context, path):
         try:
             with self._open(context, path):
                 return True
-        except (FileNotFoundError, NotADirectoryError):
+        except (FileNotFoundError, IsADirectoryError):
             return False
 
     @pass_context

_shared/project/pyproject.toml

@@ -78,6 +78,17 @@ show_missing = true
 precision = 2
 fail_under = 100.00
 skip_covered = true
+exclude_also = [
+{% if include_exists("coverage/exclude_also") %}
+    {{- include("coverage/exclude_also", indent=4) -}}
+{% else %}
+    # `if TYPE_CHECKING:` blocks are only executed while running mypy.
+    "if TYPE_CHECKING:",
+{% if include_exists("coverage/exclude_also/tail") %}
+    {{- include("coverage/exclude_also/tail", indent=4) -}}
+{% endif %}
+{% endif %}
+]
 
 [tool.isort]
 multi_line_output = 3