This replaces `mitogen.master.scan_code_imports()` with
`mitogen.imports.codeobj_imports()`. The Python 3.x implementation now uses
`str.find()`, relying on Python >= 3.6 "widecode" format. Behaviour and
semantics should be unchanged. Now implementations are approx
- 1.5 x faster on Python 2.x
- 2 - 3 x faster on Python 3.x
Before
```console
$ ./tests/bench/scan_code
scan_code_imports python2.7 100 loops, best of 3: 3.19 msec per loop
scan_code_imports python3.9 500 loops, best of 5: 685 usec per loop
scan_code_imports python3.10 500 loops, best of 5: 727 usec per loop
scan_code_imports python3.11 500 loops, best of 5: 601 usec per loop
scan_code_imports python3.12 500 loops, best of 5: 609 usec per loop
scan_code_imports python3.13 500 loops, best of 5: 586 usec per loop
```
After
```console
codeobj_imports python2.7 1000 loops, best of 3: 1.98 msec per loop
codeobj_imports python3.9 1000 loops, best of 5: 302 usec per loop
codeobj_imports python3.10 1000 loops, best of 5: 297 usec per loop
codeobj_imports python3.11 1000 loops, best of 5: 243 usec per loop
codeobj_imports python3.12 1000 loops, best of 5: 278 usec per loop
codeobj_imports python3.13 1000 loops, best of 5: 259 usec per loop
```
```console
$ uname -a
Darwin kintha 24.6.0 Darwin Kernel Version 24.6.0: Mon Jul 14 11:30:29 PDT
2025; root:xnu-11417.140.69~1/RELEASE_ARM64_T6000 arm64
```
```console
$ ./tests/bench/scan_code
scan_code_imports python2.7 100 loops, best of 3: 3.19 msec per loop
scan_code_imports python3.9 500 loops, best of 5: 685 usec per loop
scan_code_imports python3.10 500 loops, best of 5: 727 usec per loop
scan_code_imports python3.11 500 loops, best of 5: 601 usec per loop
scan_code_imports python3.12 500 loops, best of 5: 609 usec per loop
scan_code_imports python3.13 500 loops, best of 5: 586 usec per loop
```
Alex Willmer