Fix import issues.

httpcore/_async/__init__.py

@@ -7,7 +7,13 @@
 try:
     from .http2 import AsyncHTTP2Connection
 except ImportError:  # pragma: nocover
-    pass
+
+    class AsyncHTTP2Connection:  # type: ignore
+        def __init__(self, *args, **kwargs) -> None:  # type: ignore
+            raise RuntimeError(
+                "Attempted to use http2 support, but the `h2` package is not "
+                "installed. Use 'pip install httpcore[http2]'."
+            )
 
 
 __all__ = [

httpcore/_sync/__init__.py

@@ -7,7 +7,13 @@
 try:
     from .http2 import HTTP2Connection
 except ImportError:  # pragma: nocover
-    pass
+
+    class HTTP2Connection:  # type: ignore
+        def __init__(self, *args, **kwargs) -> None:  # type: ignore
+            raise RuntimeError(
+                "Attempted to use http2 support, but the `h2` package is not "
+                "installed. Use 'pip install httpcore[http2]'."
+            )
 
 
 __all__ = [

setup.py

@@ -53,7 +53,12 @@ def get_packages(package):
     packages=get_packages("httpcore"),
     include_package_data=True,
     zip_safe=False,
-    install_requires=["h11>=0.11,<0.13", "sniffio==1.*", "anyio==3.*"],
+    install_requires=[
+        "h11>=0.11,<0.13",
+        "sniffio==1.*",
+        "anyio==3.*",
+        "certifi",
+    ],
     extras_require={
         "http2": ["h2>=3,<5"],
     },