Fix TypeError when importing pytest on Python 3.5.0 and 3.5.1

The typing module on these versions have these issues:

- `typing.Pattern` cannot appear in a Union since it is not considered a
  class.

- `@overload` is not supported in runtime. (On the other hand, mypy
  doesn't support putting it under `if False`, so we need some runtime
  hack).

Refs pytest-dev#5751.

changelog/5751.bugfix.rst

@@ -0,0 +1 @@
+Fixed TypeError when importing pytest on Python 3.5.0 and 3.5.1.

src/_pytest/_code/code.py

@@ -8,7 +8,7 @@
 from types import TracebackType
 from typing import Generic
 from typing import Optional
-from typing import Pattern
+from typing import Pattern  # noqa: F401 (used in type string)
 from typing import Tuple
 from typing import TypeVar
 from typing import Union
@@ -591,7 +591,7 @@ def getrepr(
         )
         return fmt.repr_excinfo(self)
 
-    def match(self, regexp: Union[str, Pattern]) -> bool:
+    def match(self, regexp: Union[str, "Pattern"]) -> bool:
         """
         Check whether the regular expression 'regexp' is found in the string
         representation of the exception using ``re.search``. If it matches

src/_pytest/python_api.py

@@ -1,6 +1,7 @@
 import inspect
 import math
 import pprint
+import sys
 from collections.abc import Iterable
 from collections.abc import Mapping
 from collections.abc import Sized
@@ -14,7 +15,7 @@
 from typing import Generic
 from typing import Optional
 from typing import overload
-from typing import Pattern
+from typing import Pattern  # noqa: F401 (used in type string)
 from typing import Tuple
 from typing import TypeVar
 from typing import Union
@@ -28,6 +29,12 @@
 if False:  # TYPE_CHECKING
     from typing import Type  # noqa: F401 (used in type string)
 
+if sys.version_info <= (3, 5, 1):
+
+    def overload(f):  # noqa: F811
+        return f
+
+
 BASE_TYPE = (type, STRING_TYPES)
 
 
@@ -547,12 +554,12 @@ def _is_numpy_array(obj):
 def raises(
     expected_exception: Union["Type[_E]", Tuple["Type[_E]", ...]],
     *,
-    match: Optional[Union[str, Pattern]] = ...
+    match: Optional[Union[str, "Pattern"]] = ...
 ) -> "RaisesContext[_E]":
     ...  # pragma: no cover
 
 
-@overload
+@overload  # noqa: F811
 def raises(
     expected_exception: Union["Type[_E]", Tuple["Type[_E]", ...]],
     func: Callable,
@@ -563,10 +570,10 @@ def raises(
     ...  # pragma: no cover
 
 
-def raises(
+def raises(  # noqa: F811
     expected_exception: Union["Type[_E]", Tuple["Type[_E]", ...]],
     *args: Any,
-    match: Optional[Union[str, Pattern]] = None,
+    match: Optional[Union[str, "Pattern"]] = None,
     **kwargs: Any
 ) -> Union["RaisesContext[_E]", Optional[_pytest._code.ExceptionInfo[_E]]]:
     r"""
@@ -724,7 +731,7 @@ def __init__(
         self,
         expected_exception: Union["Type[_E]", Tuple["Type[_E]", ...]],
         message: str,
-        match_expr: Optional[Union[str, Pattern]] = None,
+        match_expr: Optional[Union[str, "Pattern"]] = None,
     ) -> None:
         self.expected_exception = expected_exception
         self.message = message

src/_pytest/recwarn.py

@@ -1,5 +1,6 @@
 """ recording warnings during test function execution. """
 import re
+import sys
 import warnings
 from types import TracebackType
 from typing import Any
@@ -8,7 +9,7 @@
 from typing import List
 from typing import Optional
 from typing import overload
-from typing import Pattern
+from typing import Pattern  # noqa: F401 (used in type string)
 from typing import Tuple
 from typing import Union
 
@@ -18,6 +19,11 @@
 if False:  # TYPE_CHECKING
     from typing import Type
 
+if sys.version_info <= (3, 5, 1):
+
+    def overload(f):  # noqa: F811
+        return f
+
 
 @yield_fixture
 def recwarn():
@@ -58,26 +64,26 @@ def deprecated_call(func=None, *args, **kwargs):
 def warns(
     expected_warning: Union["Type[Warning]", Tuple["Type[Warning]", ...]],
     *,
-    match: Optional[Union[str, Pattern]] = ...
+    match: Optional[Union[str, "Pattern"]] = ...
 ) -> "WarningsChecker":
     ...  # pragma: no cover
 
 
-@overload
+@overload  # noqa: F811
 def warns(
     expected_warning: Union["Type[Warning]", Tuple["Type[Warning]", ...]],
     func: Callable,
     *args: Any,
-    match: Optional[Union[str, Pattern]] = ...,
+    match: Optional[Union[str, "Pattern"]] = ...,
     **kwargs: Any
 ) -> Union[Any]:
     ...  # pragma: no cover
 
 
-def warns(
+def warns(  # noqa: F811
     expected_warning: Union["Type[Warning]", Tuple["Type[Warning]", ...]],
     *args: Any,
-    match: Optional[Union[str, Pattern]] = None,
+    match: Optional[Union[str, "Pattern"]] = None,
     **kwargs: Any
 ) -> Union["WarningsChecker", Any]:
     r"""Assert that code raises a particular class of warning.
@@ -207,7 +213,7 @@ def __init__(
         expected_warning: Optional[
             Union["Type[Warning]", Tuple["Type[Warning]", ...]]
         ] = None,
-        match_expr: Optional[Union[str, Pattern]] = None,
+        match_expr: Optional[Union[str, "Pattern"]] = None,
     ) -> None:
         super().__init__()