dev: Add type stubs for boto3 and botocore

Without these type stubs, new failures related to these libraries are
raised by Pyright starting with 1.1.309:

    …/nextstrain/cli/remote/s3.py
      …/nextstrain/cli/remote/s3.py:213:39 - error: Cannot access member "Bucket" for type "_"
        Member "Bucket" is unknown (reportGeneralTypeIssues)
    …/nextstrain/cli/runner/aws_batch/s3.py
      …/nextstrain/cli/runner/aws_batch/s3.py:208:26 - error: "client" is not a known member of "None" (reportOptionalMemberAccess)
      …/nextstrain/cli/runner/aws_batch/s3.py:212:24 - error: Cannot access member "Bucket" for type "_"
        Member "Bucket" is unknown (reportGeneralTypeIssues)
    3 errors, 0 warnings, 0 informations

With these type stubs, those failures are resolved, but different
failures are raised:

    …/nextstrain/cli/remote/s3.py
      …/nextstrain/cli/remote/s3.py:251:23 - error: Could not access item in TypedDict
        "Error" is not a required key in "_ClientErrorResponseTypeDef", so access may result in runtime exception (reportTypedDictNotRequiredAccess)
      …/nextstrain/cli/remote/s3.py:251:23 - error: Could not access item in TypedDict
        "Code" is not a required key in "_ClientErrorResponseError", so access may result in runtime exception (reportTypedDictNotRequiredAccess)
    2 errors, 0 warnings, 0 informations

All failures are fixed in this commit.

Resolves: <#284>

nextstrain/cli/remote/s3.py

@@ -248,7 +248,7 @@ def exists(object: S3Object) -> bool:
         object.load()
         return True
     except ClientError as error:
-        if 404 == int(error.response['Error']['Code']):
+        if 404 == int(error.response.get("Error", {}).get("Code", 0)):
             return False
         else:
             raise

setup.py

@@ -138,6 +138,8 @@ def find_namespaced_packages(namespace):
             "sphinx-autobuild",
             "sphinx-markdown-tables !=0.0.16",
             "sphinx_rtd_theme",
+            "types-boto3",
+            "types-botocore",
             "types-docutils",
             "types-setuptools",
             "types-requests; python_version != '3.6'",