Python `TypeError` when trying to add a `sourceMappingURL` section to a WASM file

Please include the following in your bug report:

**Version of emscripten/emsdk:**

```
emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 3.1.6 ()
Debian clang version 14.0.6
Target: wasm32-unknown-emscripten
Thread model: posix
InstalledDir: /usr/bin
```

I'm also using `Python 3.11.2`.

**Failing command line in full:**

When running this command:

```sh
/usr/share/emscripten/tools/wasm-sourcemap.py --dwarfdump-output main.dwarf main.wasm --output main.wasm.sourcemap -u http://example.com/main.wasm.sourcemap -w main-out.wasm
```

I get this output:

```
Traceback (most recent call last):
File "/usr/share/emscripten/tools/wasm-sourcemap.py", line 356, in
sys.exit(main())
^^^^^^
File "/usr/share/emscripten/tools/wasm-sourcemap.py", line 343, in main
wasm = append_source_mapping(wasm, options.source_map_url)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/share/emscripten/tools/wasm-sourcemap.py", line 142, in append_source_mapping
section_content = encode_uint_var(len(section_name)) + section_name + encode_uint_var(len(url)) + url
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~
TypeError: can't concat str to bytes
```

This error happens because the following line tries to concatenate `section_name` and `url` with values of type `bytes`.

https://github.com/emscripten-core/emscripten/blob/493649dc4caa19d75d9650988137810d4ed80eb7/tools/wasm-sourcemap.py#L143

To fix this one could simply encode the two strings and get their values as bytes like so:

```python
section_content = (
encode_uint_var(len(section_name))
+ bytes(section_name, encoding="utf-8")
+ encode_uint_var(len(url))
+ bytes(url, encoding="utf-8")
)
```

I've tested my proposed fix locally and it seems to work as intended.