fix(template): update mypy version of pyproject.toml

mypy 0.770 is not compatible with python 3.9. The issue was fixed in
0.780 and 0.800 starts to support python 3.9 officially.

mypy issue 8614 and 8627
python/mypy#8614
python/mypy#8627

Author: tai271828

{{cookiecutter.project_slug}}/pyproject.toml

@@ -114,7 +114,7 @@ coverage = {extras = ["toml"], version = "^5.0.4"}
 # style
 flake8 = "^3.7.9"
 pylint = "^2.4.4"
-mypy = "^0.770"
+mypy = "^0.800"
 black = "^19.10b0"
 isort = "^5.0.0"
 # security