Regression: inconsistent line endings

[radoering]

#149 introduced a regression especially for Windows users. If your toml file contains only windows line endings ("\r\n"), you may get mixed line endings when you read, change and write the toml file.

Example:

toml_path = str(tmpdir / "pyproject.toml")
with open(toml_path, "wb+") as f:
    f.write(b"a = 1\r\nb = 2\r\n")

f = TOMLFile(toml_path)
content = f.read()
content["c"] = 3
f.write(content)

with open(toml_path, "rb") as f:
    print(f.read())

Output: b'a = 1\r\nb = 2\r\nc = 3\n' (the last line ending is "\n" instead of "\r\n")