Fix toolchain to nightly-2023-07-28

[MichaelKohler]

There seems to be an issue with the latest nightlies:

error[E0432]: unresolved import `proc_macro::LineColumn`     
 --> /home/mkohler/.cargo/registry/src/index.crates.io-6f17d22bba15001f/inline-python-macros-0.11.0/src/embed_python.rs:1:18
  |
1 | use proc_macro::{LineColumn, Span};                                                
  |                  ^^^^^^^^^^ no `LineColumn` in the root      
  |                                                                                    
  = help: consider importing this struct instead:         
          proc_macro2::LineColumn

error[E0615]: attempted to take value of method `column` on type `proc_macro::Span`
  --> /home/mkohler/.cargo/registry/src/index.crates.io-6f17d22bba15001f/inline-python-macros-0.11.0/src/embed_python.rs:69:14
   |                                                                                   
69 |                     end_loc.column = end_loc.column.saturating_sub(end.len());
   |                             ^^^^^^ method, not a field
   |                                                                                   
   = help: methods are immutable and cannot be assigned to             

.....