Simplify python3.dll build

[nnemkin]

BPO	41039
Nosy	@pfmoore, @tjguk, @zware, @eryksun, @zooba, @nnemkin
PRs	bpo-41039: Simplify python3.dll build #20989

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/zooba'
closed_at = <Date 2020-06-23.19:34:26.248>
created_at = <Date 2020-06-19.16:27:16.218>
labels = ['type-feature', '3.10', 'OS-windows']
title = 'Simplify python3.dll build'
updated_at = <Date 2020-06-23.19:34:26.247>
user = 'https://github.com/nnemkin'

bugs.python.org fields:

activity = <Date 2020-06-23.19:34:26.247>
actor = 'steve.dower'
assignee = 'steve.dower'
closed = True
closed_date = <Date 2020-06-23.19:34:26.248>
closer = 'steve.dower'
components = ['Windows']
creation = <Date 2020-06-19.16:27:16.218>
creator = 'nnemkin'
dependencies = []
files = []
hgrepos = []
issue_num = 41039
keywords = ['patch']
message_count = 5.0
messages = ['371894', '372115', '372130', '372199', '372200']
nosy_count = 6.0
nosy_names = ['paul.moore', 'tim.golden', 'zach.ware', 'eryksun', 'steve.dower', 'nnemkin']
pr_nums = ['20989']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue41039'
versions = ['Python 3.10']

[nnemkin]

python3.dll build process can be simplified if we use linker comment #pragma instead of .def files. Custom build targets become unnecessary and hardcoded Python DLL name can be replaced with a macro.

Also, python3.dll doesn't need DllMain and can be built with /NOENTRY.

[zooba]

Thanks for the change. It looks good, though I would like to have a NEWS entry for this so that people can find that we made a deliberate change to this DLL.

I'd also like to do some manual testing to make sure that we aren't going to break any obscure uses. That's basically blocked on me, sorry, but I'll try and get to it asap. Feel free to ping me in a week if I haven't gotten to it.

[nnemkin]

Please take your time.

FWIW, I tested this patch by 1) comparing dumpbin /exports output; 2) comparing bunaries themselves (.dll and .lib) and 3) linking xxlmited module.

It helps that 7-zip can open .dll and .lib files files as if they were archives.

[zooba]

New changeset 2c6e4e9 by Nikita Nemkin in branch 'master':
bpo-41039: Simplify python3.dll build (GH-20989)
2c6e4e9

[zooba]

Thanks!