Update cortex-m requirement from 0.6.1 to 0.7.3

[dependabot-preview[bot]]

Updates the requirements on cortex-m to permit the latest version.

Release notes

Sourced from cortex-m's releases.

v0.7.3

[v0.7.3] - 2021-07-03

Fixed

• Fixed compilation for native targets on non-x86 host systems (#336, #337).

Added

• The Delay struct now offers direct delay_us() and delay_ms() methods without having to go through the embedded-hal traits (#344).

Changelog

Sourced from cortex-m's changelog.

[v0.7.3] - 2021-07-03

Fixed

• Fixed compilation for native targets on non-x86 host systems (#336, #337).

Added

• The Delay struct now offers direct delay_us() and delay_ms() methods without having to go through the embedded-hal traits (#344).

[v0.7.2] - 2021-03-07

Fixed

• Fixed a bug where calling asm::delay() with an argument of 0 or 1 would underflow, leading to a very long delay.

[v0.7.1] - 2021-01-25

Added

• New assembly methods asm::semihosting_syscall, asm::bootstrap, and asm::bootload.

Deprecated

• msp::write has been deprecated in favor of asm::bootstrap. It was not possible to use msp::write without causing Undefined Behavior, so all existing users are encouraged to migrate.

Fixed

• Fixed a bug in asm::delay which could lead to incorrect codegen and infinite loops.
• Improved timing guarantees of asm::delay on multiple-issue CPU cores.
• Additional compiler fences added to inline assembly where necessary.
• Fixed DWARF debug information in pre-built assembly binaries.

[v0.7.0] - 2020-11-09

Added

• New InterruptNumber trait is now required on interrupt arguments to the various NVIC functions, replacing the previous use of Nr from bare-metal. For backwards compatibility, InterruptNumber is implemented for types which are Nr + Copy, but this will be removed in a future version.
• Associated const PTR is introduced to Core Peripherals to eventually replace the existing ptr() API.
• A delay driver based on SysTick.

... (truncated)

Commits

• 45e7ab7 Merge #345
• 1747d44 Prepare for v0.7.3.
• 47246ec Merge #344
• 4a393e0 Update src/delay.rs
• 8133657 Use from instead of as
• 7329f57 remove inline on delay_us, and made u32 conversion syntax consistent
• d13ebd2 Use . syntax instead of :: on delay_ms
• 1a2b6ea Use overflow-workaround on delay_ms
• cf5dbcd Remove dependence on embedded-hal for Delay
• 1a4e096 Merge #341
• Additional commits viewable in compare view

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