[clang-tidy] Always open files using UTF-8 encoding

The encoding used for opening files depends on the OS and might be different
from UTF-8 (e.g. on Windows it can be CP-1252). The documentation files use
UTF-8 and might be incompatible with other encodings. For example, right now
`clang-tools-extra/docs/clang-tidy/checks/abseil-no-internal-dependencies.rst`
has non-ASCII quotes and running `add_new_check.py` fails on Windows, because
it tries to read the file with incompatible encoding.

Use `io.open` for compatibility with both Python 2 and Python 3.

Reviewed By: kbobyrev

Differential Revision: https://reviews.llvm.org/D106792

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