Python-Dotenv Files

Fixed

• Strip a leading UTF-8 BOM from .env file contents so the first variable is no longer silently lost when the file is saved with BOM (e.g. by some JetBrains IDEs on Windows) by [@h1whelan] in [#640]
• set_key now escapes backslashes, so values containing them (Windows paths, regular expressions) survive a write/read round-trip. Quoted values ending in an escaped backslash are no longer mis-parsed as an escaped quote, which used to swallow the following lines by [@dchaudhari7177] in [#680]
• dotenv run now prints a friendly error instead of a traceback when no command is given by [@bbc2] in [#606]
• Cache the parsed result for empty .env files so repeated dotenv_values/load_dotenv calls no longer re-read the file by [@ReinerBRO] in [#638]