Convert Windows-style newline (\r\n) characters to Unix-style (\n)

[vladimir-v-diaz]

Fixes issue #:

Address theupdateframework/python-tuf#709 and theupdateframework/python-tuf#690.

Description of the changes being introduced by the pull request:

This pull request converts \r\n newline characters (in keys data) to \n so that consistent keyids are generated for public keys that differ only in the style of newline characters.

For instance, an RSA PEM string that is generated in Windows might contain \r\n characters. These newline characters would need to be read/written in a consistent manner so that the keyid of the PEM string matches the one generated on MacOS/Linux (which might treat load these newline characters as \n.

Before generating the keyid of public key data (e.g., RSA PEM string), this pull request applies replace('\r\n', '\n) to the string.

Please verify and check that the pull request fulfills the following
requirements:

• The code follows the Code Style Guidelines
• Tests have been added for the bug fix or new feature
• Docs have been added for the bug fix or new feature

Signed-off-by: Vladimir Diaz <vladimir.v.diaz@gmail.com>