fix(backups): setup backups for bench sites

[gavindsouza]

Using python-crontab library instead of manual processing

INFO:bench.utils:setting up backups
no crontab for revant
SUCCESS: Bench fbench initialized
crontab: usage error: file name or - (for stdin) must be specified for replace
Usage:
 crontab [options] file
 crontab [options]
 crontab -n [hostname]

Options:
 -u <user>  define user
 -e         edit user's crontab
 -l         list user's crontab
 -r         delete user's crontab
 -i         prompt before deleting
 -n <host>  set host in cluster to run users' crontabs
 -c         get host in cluster to run users' crontabs
 -V         print version and exit
 -x <mask>  enable debugging

Default operation is replace, per 1003.2

Reference: #967 (comment)

[revant]

❯ cd /tmp
❯ bench init --skip-redis-config-generation fbench --skip-assets
WARN: bench is installed in editable mode!

This is not the recommended mode of installation for production. Instead, install the package from PyPI with: `pip install frappe-bench`

WARN: Command not being executed in bench directory
$ /home/revant/.local/bin/virtualenv -q env -p python3
INFO:bench.app:Getting app frappe
$ git clone https://github.com/frappe/frappe.git  --depth 1 --origin upstream
Cloning into 'frappe'...
remote: Enumerating objects: 2737, done.
remote: Counting objects: 100% (2737/2737), done.
remote: Compressing objects: 100% (2509/2509), done.
remote: Total 2737 (delta 397), reused 1052 (delta 164), pack-reused 0
Receiving objects: 100% (2737/2737), 12.83 MiB | 1.04 MiB/s, done.
Resolving deltas: 100% (397/397), done.
INFO:bench.app:installing frappe
$ fbench/env/bin/pip install -q -U -e fbench/apps/frappe 
INFO:bench.utils:setting up backups
Please specify user or filename to write.
ERROR: There was a problem while creating fbench
Do you want to rollback these changes? [Y/n]: 

[revant]

❯ bench init --skip-redis-config-generation fbench --skip-assets
WARN: bench is installed in editable mode!

This is not the recommended mode of installation for production. Instead, install the package from PyPI with: `pip install frappe-bench`

WARN: Command not being executed in bench directory
$ /home/revant/.local/bin/virtualenv -q env -p python3
INFO:bench.app:Getting app frappe
$ git clone https://github.com/frappe/frappe.git  --depth 1 --origin upstream
Cloning into 'frappe'...
remote: Enumerating objects: 2739, done.
remote: Counting objects: 100% (2739/2739), done.
remote: Compressing objects: 100% (2511/2511), done.
remote: Total 2739 (delta 396), reused 1050 (delta 164), pack-reused 0
Receiving objects: 100% (2739/2739), 12.83 MiB | 123.00 KiB/s, done.
Resolving deltas: 100% (396/396), done.
INFO:bench.app:installing frappe
$ fbench/env/bin/pip install -q -U -e fbench/apps/frappe 
INFO:bench.utils:setting up backups
SUCCESS: Bench fbench initialized

LGTM